Did the Allies ever seriously contemplate a "Northern Intervention" in Russia during World War II?

British Responses to the threat of Soviet Collapse.

The Grand Alliance
After the beginning of Operation Barbarossa on 22nd June 1941, the British were well aware of the strategic importance to themselves of keeping the Soviet Union fighting for as long as possible. Churchill announced to the world on the day of the invasion the British government's intention to support the USSR in every way possible to fight their common enemy. He later wrote:

The entry of Russia into the war was welcome but not immediately helpful to us. The German armies were so strong that it seemed that for many months they could maintain the invasion threat against England while at the same time plunging into Russia. Almost all responsible military opinion held that the Russian armies would soon be defeated and largely destroyed.

...

The strength of the Soviet government, the fortitude of the Russian people, their immeasurable reserves of man-power, the vast size of their country, the rigours of the Russian winter, were the factors which ultimately ruined Hitler's armies. But none of these made themselves apparent in 1941. President Roosevelt was considered very bold when he proclaimed in September 1941 that the Russian front would hold and that Moscow would not be taken. The glorious strength and patriotism of the Russian people vindicated this opinion.

Even in August 1942, after my visit to Moscow and the conferences there, General Brooke, who had accompanied me, adhered to the opinion that the Caucasus Mountains would be traversed and the basin of the Caspian dominated by German forces, and we prepared accordingly on the largest possible scale for a defensive campaign in Syria and Persia. Throughout I took a more sanguine view than my military advisers of the Russian powers of resistance. I rested with confidence upon Premier Stalin's assurance, given to me at Moscow, that he would hold the line of the Caucasus and the Germans would not reach the Caspian in any strength. But we were vouchsafed so little information about Soviet resources and intentions that all opinions either way were hardly more than guesses.

_{Churchill, Vol.III, (pp. 350-351)}

Shipping Aid to Russia
In immediate response to the invasion, convoys of supplies and military equipment for Russia were organized, diverting invaluable resources away from the already stretched British military forces to the Soviets, and by September British warships and planes were deployed into the Arctic, basing from Spitzbergen, Murmansk, and Archangel, to protect British convoys and support the Soviet navy.

Preparing for the Worst
In early July 1941, urgent demands were made by the War Cabinet on the new British commander in the Middle East, General Auchinleck, to advance plans for a counter-attack (Operation CRUSADER) to recapture key airfields in Cyrenaica which were important for interdicting German reinforcements into Africa, and to bring the Syrian Campaign to a conclusion and secure the Near East before the collapse of Russia and the inevitable surge of German activity against Britain which would follow. (Churchill, Vol. III)

Prospects for Military Co-operation
On 15th September 1941, just a few weeks after the joint Anglo-Soviet occupation of Iran opened up a new line of communication to Russia in the Near East, Churchill received a telegram from Stalin containing the following perhaps surprising passage:

"I have no doubt that the British government desires to see the Soviet Union victorious, and is looking for ways and means to attain this end. If, as they think, the establishment of a second front in the West is at present impossible, perhaps another method could be found to render to the Soviet Union an active military help?

"It seems to me that Great Britain could without risk land in Archangel twenty-five to thirty divisions, or transport them across Iran to the southern regions of the USSR. In this way there could be established military collaboration between the Soviet and British troops on the territory of the USSR. A similar situation existed during the last war in France. The arrangement mentioned would constitute a great help. It would be a serious blow against the Hitler aggression."

_{Churchill, Vol.III, (p. 411)}

To which Churchill observed, "It is almost incredible that the head of the Russian government with all the advice of their military experts could have committed himself to such absurdities. It seemed hopeless to argue with a man thinking in terms of utter unreality."

His formal reply to Stalin, on 17th September 1941, was somewhat more diplomatic:

"All possible theatres in which we might effect military co-operation with you have been examined by the Staffs. The two flanks, north and south, certainly present the most favourable opportunities. If we could act successfully in Norway the attitude of Sweden would be powerfully affected, but at the moment we have neither the forces nor the shipping available for this project. Again, in the south the great prize is Turkey; if Turkey can be gained another powerful army will be available. Turkey would like to come with us, but is afraid, not without reason. It may be that the promise of considerable British forces and supplies of technical material in which the Turks are deficient will exercise a decisive influence upon them. We will study with you any other form of useful aid, the sole object being to bring the maximum force against the common enemy."

_{Churchill, Vol.III, (p. 412)}

British Forces in Russia?
Nevertheless, British military command were at this time expecting German forces to reach the Caucasus sometime around November 1941-January 1942 if the Soviet front collapsed, and they wished to deploy a 2-3 division British corps on a mountain line in front of Baku. The British 50th and 18th Divisions were assigned to this task and were prepared for shipment to the Middle East with their arrival in Baku expected before January. Ten squadrons of aircraft were also assigned to support the force around Baku. An Indian division was to follow later. The existing Indian divisions in theatre had been deemed unsuitable for the operation. This was the maximum force which could be supplied over the road network, with the rail lines being reserved for Soviet supplies. Transport limitations delayed the arrival of the British divisions beyond March and ultimately Churchill opposed the movement of British forces into Russia. He claimed the Russians did not need more troops, that supplies were more important, and he preferred to relieve the Soviet garrison in Persia as an alternative, an idea which was quickly rejected by Molotov. (Gwyer, Vol. III, Part 1)

On 25th October 1941, Churchill shared some of his thoughts with the British ambassador Sir Stafford Cripps in Moscow:

"You were of course right to say that the idea of sending "twenty-five to thirty divisions to fight on the Russian front" is a physical absurdity. It took eight months to build up ten divisions in France, across the Channel, when shipping was plentiful and U-boats few. It is with the greatest difficulty that we have managed to send the 50th Division to the Middle East in the last six months. We are now sending the 18th Division by extraordinary measures. All our shipping is fully engaged, and any saving can only be made at the expense of our vital upkeep convoys to the Middle East or of ships engaged in carrying Russian supplies. The margin by which we live and make munitions of war has only narrowly been maintained. Any troops sent to Murmansk now would be frozen in darkness for the winter.

"Position on the southern flank is as follows: Russians have five divisions in Persia, which we are willing to relieve. Surely these divisions should defend their own country before we choke one of the only supply lines with the maintenance of our forces to the northward. To put two fully armed British divisions from here into the Caucasus or north of the Caspian would take at least three months. They would then only be a drop in the bucket."

_{Churchill, Vol.III, (p. 413)}

British Interests First!
By early 1942, in expectation of a renewed German summer offensive in the Soviet Union, British forces in Iraq and Persia were transferred from India Command to Middle East Command and re-designated as the British 10th Army charged with the defense of the vital oil fields of the Near East and the lines of communication to Russia. Development of ports, railways and roads were handed over to American control, and defensive positions and airfields were constructed. Commitments of British air support were made to both Turkey and Russia if required to meet the renewed German threat and first contact with German forces in Persia was anticipated around mid-August 1942, with a possible major effort in Spring 1943. In August 1942 a new Persia and Iraq Command was created to allow Middle East Command to focus on the defense of the Nile Delta and the Suez Canal, with Abadan, now situated within the new Command, recognized as having higher strategic importance than Egypt (Playfair, Vol III). The defeat of the Germans at Stalingrad and the successful Soviet defense of the Caucasus had removed the German threat to British interests in the Near East by early 1943 and the likelihood of Soviet defeat in the war receded significantly after that time.

Conclusion
So from the British perspective, in answer to the question posed, it does not appear that the British were ever seriously intending to intervene physically into Soviet territory on the northern flank during the period when Soviet survival was uncertain. On the contrary, the British seem to have regarded operations in the Arctic north as wildly impractical. In the south they did seriously consider a minor screening operation in front of Baku, but ultimately they preferred to support Soviet troops with supplies and were relying on the poor communications through the Caucasus area to limit German incursions and thereby aid the defense of their own vital interests beyond.

U.S. Responses to the Threat of Soviet Collapse.

Recognizing the Mortal Danger of German Hegemony
Across the Atlantic, the government of the United States, still maintaining neutrality, but slowly edging towards greater participation in the war, was also re-examining their response to the developing global strategic situation after observing the progress of the German invasion of the USSR. Initially there was almost unanimous acceptance in American circles that the Soviet Union would be quickly defeated within just a few weeks or months. On 18th July 1941, US War Department G-2 intelligence analysts predicted German occupation of Russia as far as Lake Baikal or the Pacific coast definitely by the end of 1941. However as events unfolded a view emerged that the Soviets could potentially hold out for longer. After Harry Hopkins' visit to Moscow in August 1941, he and the American military attache Col. Philip Faymonville became convinced that the Soviet Union would continue to resist and would survive the German invasion, and President Roosevelt accepted their advice. Thereafter preserving and prolonging Soviet resistance became seen as a vital US interest, and maintaining constructive relations with the Soviet government became an important political consideration (Stoler p.50-54).

Mobilizing for Victory
On 18th August 1941, the service duration for conscription into the US peacetime Selective Service System was extended from 12 months to 30 months. The Victory Plan, an updated assessment of future military requirements, was issued in September 1941 with a declared national goal of eliminating totalitarianism from Europe. The great strategic concern facing the United States was the possibility that Germany might occupy and consolidate a dominant position across the entire Eurasian land-mass. Kirkpatrick explains that "such a situation would present the United States with the most difficult military problem imaginable, particularly if it were compounded by the catastrophe of the fall of the British Isles. In that case the nation would have lost the only remaining area in Europe from which it could conduct effective operations against Germany." (Kirkpatrick, p.72)

The British faced enormous risk, however, and G-2 analyses could not confidently predict victory for the United Kingdom, even with full American collaboration. British reverses in the Middle East, or a Russian collapse on that front, would enable the Germans to concentrate an overwhelming military force against England. For the British, the situation hinged on three issues: the German ability to win quickly in Russia without suffering excessive losses; the German ability to reconstitute military forces quickly after that victory; and the German ability to control the conquered regions and exploit their resources with the use of minimal forces. Having outlined such grim prospects, Smith concluded that "from a long range viewpoint, the situation is not hopeless for Great Britain, assuming the continuation of Russian resistance and/or full U.S. participation in the war."

...

The health of Russia was therefore of paramount concern, and the Soviet situation defined the time available for the United States to act against Germany. If Russia lost the war by the end of 1941, the Germans would probably require one full year to reorganize their armed forces to conduct an invasion of the British Isles. Germany would likely also need a full year to bring sufficient order out of the chaos of the conquered territories to be able to benefit militarily and economically from them. The earliest, therefore, that the Axis could mount an invasion of England would be the spring of 1942, with the spring of 1943 a much more likely date. In the meanwhile, the United States needed to provide for the security of the western hemisphere in the event that Russia collapsed and the British suffered invasion or agreed to negotiate a peace.

_{'An Unknown Future and a Doubtful Present: Writing the Victory Plan of 1941', Charles E. Kirkpatrick (pp. 71-72)}

Whatever it Takes to Keep Russia Fighting
In October 1941, Lend Lease was formally extended to the USSR, although American supplies and equipment had already been diverted by Britain on their Arctic convoys to the Soviet Union from as early as the first week after the German invasion. The US War Planning Division (WPD) and G-2 agreed in October 1941 that "the most potent factor in the weakening of Axis war potential is continued active operations on the Russian front. Every effort must be made to prolong this campaign. This should have first priority. Diversions in other theaters such as the Middle East, Africa, or Norway would engage only a very minor portion of Axis forces." In October the WPD also reversed its past insistence on not provoking Japan, and recommended continuation of economic pressures so as to render that nation "incapable of offensive operation against Russia" and other allies in the Pacific and Far East. G-2 went further and now began advising against US support for any settlement of the conflict between Japan and China, claiming that it was "imperative" to keep the Japanese military fully engaged in China to prevent the release of forces for use against Russia (Stoler p.55).

The Atlantic Partnership
After Pearl Harbor, full US participation in the war removed restrictions on mobilization, however their military options were constrained by a lack of readiness, and the need to co-operate with their British allies who would expect to play the dominant role in any joint operation in existing theatres.

In early 1942, the primary existing British plan to respond to an imminent collapse of Soviet resistance on the Russian front was Operation SLEDGEHAMMER, an invasion of northern France, which was intended as a diversion to draw German troops away from Russia. It was a relatively small operation, with little follow-up intended, which was not expected to survive a major German countermove if the Soviets were defeated. After the US joined the war General Marshall submitted a new expanded version of Operation SLEDGEHAMMER which was intended to be a more permanent move into France, with significant follow-up forces. However British planners were not convinced that either of these operations would divert meaningful numbers of German forces from Russia, and both would likely fail if faced with the full force of the Wehrmacht after a Russian defeat. An alternative operation in Norway (Operation JUPITER) was also deemed impractical and even less value as a diversion intended to help Russia. So in conformity with their alliance partners, the US were not looking to the northern flank of the USSR for any kind of intervention as a means to help or secure parts of Russia after their entry into the war.(Ross p.28)

The British preferred a peripheral strategy aimed at securing bases and sea lanes while they built up their forces, developing threats throughout the continent intended to stretch and disperse German logistics and resources. The US military were opposed to this approach, wanting a more direct strategy, but their lack of experienced troops and Roosevelt's urgency to have US forces engaging Germans meaningfully before the end of 1942 meant they had no choice but to conform with the British approach, and so it was to the south where initial US efforts were to be directed. (Stoler, p.51)

Securing The Foundations
The initial success of the German offensive in Russia in 1942 required a response and the US were faced with two realistic options; to launch Operation GYMNAST and invade French North Africa, or to send troops to the Middle East to assist British forces there. The possibility of sending two US army corps to the Middle East was discussed, providing two US armoured divisions to join British forces in Iraq and Persia was considered, and the US assumed responsibility for building and maintaining ports, roads, and rail in the Near East. They also developed an air base in Abadan in Persia, and a port and an airbase for bombers at Massawa on the Red Sea, laying the foundation for an expanded presence in the Middle East or Near East should the need arise (Playfair, Vol. III). However the decision was ultimately made to execute the GYMNAST plan, now named Operation TORCH, and invade French North Africa, in order to progress the securing of the Atlantic and the Middle East/Mediterranean, to provide for American communications directly into the European theatre, and to help the Soviets by significantly expanding the threat to the Axis powers across southern Europe.

Preparing for the Worst
The Japanese entry into the war complicated the commitment the US had made to their British ally to prioritize the European theatre with a Germany First strategy. After the treachery of Pearl Harbor there was considerable pressure from the public and from within the US military for greater emphasis on the war against Japan, however there was general agreement that a hegemonic Germany posed a significantly greater threat to US interests broadly. Nevertheless, despite maintaining their focus on Germany, the US preferred to keep secret from the British their plan to deal with a complete collapse of Soviet resistance, believing the British would perceive it as a renunciation of the Germany First principle (Stoler, p.95). This suggests, without having further details of the US plans available, that the US were intending to heavily shift their attention to the Asian theatre in the event of total Soviet collapse (Stoler, pp.80-83), possibly using India and China as bases for expanding operations against German forces in the USSR, after eliminating the threat from Japan. In this case of total Soviet defeat, which may be hard to imagine with hindsight today (but may not have seemed unlikely at all to many within the US leadership circle at the time), it seems likely that the US could indeed have been required to send major combat forces deep into the territory of the USSR to engage and defeat Germany head-on in the heartland of Eurasia, with or without the support of the remnants of the defeated Soviet military. Perhaps when General Wedemeyer's Victory Plan of September 1941 (which proposed a US army of 215 divisions) declared the US goal in going to war was to "eliminate totalitarianism from Europe" (Kirkpatrick, p.63), he meant it.

Making America Really Great
Discussing his vision of the post-war world with Sumner Welles in August 1941, when few saw any prospect of long-term Soviet survival, President Roosevelt suggested as a proposed element of the Atlantic Charter, to commit the United States and Great Britain to a post-war policy of forcibly disarming aggressive nations and creating an 'International Police Force' to enforce global disarmament. When Welles indicated that this might include disarming the USSR, he recalled that Roosevelt simply shrugged his shoulders (Wilson, pp.174-175). Still pursuing his idea of an 'International Police Force' in late 1942, Roosevelt commissioned a report to determine suitable locations for the establishment of the necessary air bases and facilities "without regard for current sovereignty" (Stoler, p.138). Clearly it was the intention of Roosevelt, in those early uncertain days at least, to ensure that US dominance would be asserted across the globe after the war, and it can be assumed, that if in the process of defeating Germany US forces had been called upon to liberate portions of the USSR that this would have occurred in a manner designed to ensure that the USSR did not emerge as a strategic threat to the USA in the post-war world.

Conclusion
While the survival of the Soviet Union remained uncertain the United States kept every option open, and was mobilizing to ensure every means was available to prevent German domination of Eurasia. While specific interventions on Soviet soil do not appear to have been proposed, there is little doubt that had the strategic situation arisen, the US was prepared, in partnership with the British, to liberate all German occupied territory and establish a post-war strategic environment serving its interests. The decisive defeats inflicted on Germany by the Soviet Union in late 1942 and 1943, of course, changed the strategic outlook dramatically, with Soviet victory in the war increasingly certain, her emergence as a major global power in the post-war world required a new US vision which did not allow for the casual dismissal of Soviet sovereignty or interests, so the prospect of US intervention on Soviet soil from that time forward became practically unnecessary and politically unlikely.

Sources
The Second World War, Volume III: Winston Churchill
(HotSWW) Grand Strategy, Volume III, Part 1: J.M.A. Gwyer
(HotSWW) The Mediterranean and Middle East, Volume II: Maj.-Gen. I.S.O. Playfair
(HotSWW) The Mediterranean and Middle East, Volume III: Maj.-Gen. I.S.O. Playfair
United States Army in World War II, The War Department: Ray S. Cline
An Unknown Future and a Doubtful Present: Charles E. Kirkpatrick
Allies and Adversaries: Mark A. Stoler
The First Summit: Theodore A. Wilson
American War Plans 1941-1945: Steven T. Ross