What options were available to MacArthur to prepare for Chinese intervention in the Korean War?

The directive being obeyed is simply not to start a war with China. A show of force is often the first act of any commander, regardless of nation, in hopes to resolve a pending conflict before it starts. China has its reasons, and so does everyone else, but in regards to Korea, it is more often than not a war of faces, words and posturing.

The Korean leadership, on both sides of the neutral zone, are not an experienced fighting force, and are always assessing the potential losses of an all out war, As most do. its the right thing to do if it prevents senseless loss of life and liberty.

The Americans and the Chinese are always posturing and assessing each other, they both don't want a war with each other but both sides suspect it is the others intention. The same sentiment applies to Russia and the Americans. Korea is right along Russians south eastern Boarder, our western boarders. A war in Korea, Involving China and the UN is a surefire way to send this whole world to hell.

The Russians are so completely ready to go to war with everything, as they think the entire world around them is scheming against them and they have the history to prove it. The conflict between Russia and Ukraine is thousands of years old, dating back to the city stats of Stalingrad and Kiev, who were for the longest time always trying to destroy each other.

There is always someone trying to force the Rusky hand, and the same for the Americans. So when the war in Korea happened Russia had no choice but to try and support the north because it would have landed a world war right at their door step which to them happens to F$%en much and the UN trying to "Keep the Peace" had no choice but to attempt to deter the entire thing as to a avoid a war with china.

The Real loss of the entire thing is ancient Kwon Do, the root martial arts behind Taekwondo, which is Korea's national and spiritual martial-art, which is presumably the reason China was aggravated sharing its northern boarder with overly disruptive and aggressive Kwon Do practitioners. Which to china is just pure evil.

so, don't start a war with china. :p

Anyways, Korea is pincered between Russia at its North, Mongolia at is east and china and its southern boarder, so if one were to make a ground offensive of deterrence or otherwise they would have no choice but to fishbowl from the mountains, The tactic has the advantage of high ground and the disadvantage of bad terrain due to the entire region is surround by mountains.

I'm forgoing a lot.

The other options being a Western beachhead, which is just not a good idea (WW1 - Vimmy Ridge, the go to lesson here in Canada), or an air raid, which Russia would go all crazy about as they do not like our airplanes anywhere near it one bit. So from MacAurthers respective it was the best tactical option.

Besides the political and strategic solutions, that were detailed in an other answer and are not operationnal solutions nor solutions under Mac Arthur's control, here are some possibilities:

• Better intelligence and better assessment of intelligence: Proper intelligence allows to prepare for the PVA's attack, but it was hard to get: Chinese airspace could not be entered by UNO's aircraft, and I suppose there was not that much American spies in China at this period. Even more, having proper inteligence on ennemy's actions does not mean you have the means to defeat them. So I suppose intelligence could help, but is not an operationnal, fast of use and standalone solution.

Let's check for others:

• Organisation of troops: After the landing of Incheon, American forces launch a pursuit that was similar to what they achieve in Northwestern Europe in 1944-1945 (sometimes). This includes fast moving motorized columns, heavy air support and even an airborne assault that was supposed to speed up the offensive and capture North Koreans soldiers. But at the end, on the Yalu, UNO forces were quite disorganized: not that units disbanded, but they were marching on a ground they did not know, they have not enough maps nor time to recognize the roads, and they were exhausted
• Logistics: the Americans faced logistics problems when they were attacked by Chinese forces. It was not really about sending supplies to the troops (even if they met some problems with the winter and unprepared equipment and clothes), but it was about sending them on roads where they were ambushed. The UN had not enough infantry to secure all the ground, thus PVA's soldiers, using night to penetrate the lines, were able to go to the rears and attack logistics
• Besides land logistics, there was a true logistic problem for air power: The offensive was so fast that not enough attack airplanes got airstrips with enough supplies from which to operate over the Yalu. Since PV had no air power and a lot of infantry, this lack of air power was a problem because it could have performed bombing and gunning on infantrymen.

The air logistic and the organisation could have been solved, and thus UNO forces could have been strengthened, by slowing the offensive so that the logistics could follow-up. But this might not have been enough, since the problem of land logistics is still there. And it is linked to a wider issue: lack of proper doctrine.

During the Yalu offensive, UNO's forces failed to adopt proper tacticals guidelines. They were acting as motorized troops in fluid battle, but with not enough men, and facing an opponent who was not tide to the roads, it resulted in two things:

• Moving colums were ambushed, logistics as fighting columns
• Other parts of the armies, who were not supposed to fight on the move (artillery, infantry) were threatened to be cut off their rears and encircled in villages or crossroads

A a result of this two elements, UNO forces started a massive retreat, and could not establish proper defensive lines. A lot of soldiers evacuated by sea.

A solution could have been to hold the ground, everywhere, all the time: Chinese infantrymen, tough mobile and brave, had not enough infantry heavy weapons to capture an Allied defense position. It was like the Japanese in Guadalcanal, or the Russians in front of Moscow in 1942: they could go to the rears of the ennemy with infantry or cavalry, they could force the ennemy to a defensive stance. But as soon as they attacked the position, they were defeated by heavier firepower and better training. In Guadalcanal, this resulted in Americans standing for months until the logistic line of their ennemies was destroyed. But this was a sea line, so Moscow's example is more interesting: the Russian offensive started well, but as soon as the central part of the front stop retreating and entrenched itself, all Russian attacks failed. And the forces they had send fa in the rear (Belov airborne and cavalry troops for example) could move freely, but attack nothing, and they were finally destroyed by reinforcing troops.

In North Korea, this could have been done: maybe not exactly on the Yalu, but a few kilometers behind: the UNO troops showed themselves able to hold a ground (such as the Turkish corps) on a local fight, even if outnumbered. They could have created little "forts" in which waiting for attacks, with air supply and some big fighting columns occasionnally reaching them with supplies. Chinese PVA could have been blooded in those attacks, until more South Korean and American troops went to the battlefield to re-open the lines.

But the UNO troops were not ready for that: they were ordered, trained and used to fight a mobile war. So this would have required to anticipate the new situation and at least some training to the new tactics.

In real life, UNO did integrate those new tactics, but with a new commander and after having been pushed back to the South of Seoul. With those tactics, ansd their offensive twins, they were able to take back Seoul and hold the 38th parallel for 2 years.

EDIT: Adding elements after first comments of @Argyll

About the choice of the 38th parallel, the Yalu line or the Anju-Hammung line: The insights I gave above are more about tactics and logistics, which means they are time and space-dependent. However, I have not enough knowledge to give a precise idea of when and where UNO's forces could have blocked Chinese advance, provided they had adopted those measures. Still, we can think about it:

• In real world, the Yalu line was easily forced by Chinese forces: too far ahead, this line was reached only a few days before counterattack, so there was no time to organize proper defense. In the Moscow example I gave, Germans did have to fall back before standing on a new line. So Yalu is a no-go, according to me.
• In real world, after the retreat, UNO's forces lost Seul but were able to take it back and stabilize South of the South parallel. Then they only took little ground until reaching 38th parallel approximately. With appropriate measures, the standing line I describe could certainly have been hold farther north. Indeed, it even needed to be held as soon as possible since more retreat (which implies more coordination problems since Chinese are following the retreat) levels down the chance to hold strong points.

For example, when US Marines were forced to evacuate by the port of Hungnam on December 15th, they leveled down the chance to establish a defense on the 38th parallel.

About the defensive depths: I don't think a strategic-deep defense was needed, since Chinese attacked line by line the defense. However, each line should have been constructed as a series of tactical deep strong points, with forward posts, entrenched cities and artillery bases. Similar to wat was found on the Bulge in 1944.

Statement: The initial successes of Chinese military in Korean War against high odds can be said to be a major blunder on the part of MacArthur's military leadership.

The initial successes of Chinese military came because of their huge numerical advantage not as you say against high odds. On Nov. 25-26, 1950, 300,000 Chinese troops attacked American and U.N. forces in North Korea. On 1 September 1950 the United Nations Command had a strength of 180,000 in Korea: 92,000 were South Koreans, the balance being Americans and the 1,600-man British 27th Infantry Brigade.

Question:
What options were available to MacArthur to prepare for Chinese intervention in the Korean War?

China entering the Korean War was a strategic reordering episode of the war. MacArthur's permission to advance beyond the 38th parallel was prefaced with the exclusion of Chinese or Soviet involvement.

An Overview of US Army in the Korean War
Truman authorized MacArthur to send his forces north of the 38th parallel on Sept. 27, provided there was no indication that major Soviet or Chinese Communist Forces (CCF) would enter the war. The U.N. General Assembly approved the UNC's entry into North Korea 10 days later, when it called for the restoration of peace and security throughout Korea. American and ROK Army forces rapidly advanced northward.

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China's ultimate massive involvement was thus not military failure under MacArthur's control but rather a failed political gamble by policy makers. Gambling advancing into N. Korea against the larger global concerns China's involvement entailed. Washington Policy makers concerns were if china was threatened the Soviet Union and the Soviet Bomb would also enter the war. This could mean expanding the war and even potentially a Soviet Invasion of Europe. These global concerns which ultimately dictated strategies for the UN forces in Korea were outside of MacArthur's mandate.

MacArthur's ultimate dismissal as commander of UN forces in Korea had to do with MacArthur's differences with Washington on how to respond to the Chinese offensive. MacArthur sought to widen the war, exactly what Washington did not want to do. MacArthur wanted to involve Chinese Republican troops from Taiwan, Bomb Chinese staging areas inside of China, perhaps use America's nuclear weapons, and ultimately invade and wage war on China directly. Truman elected to contain the war. Block North Korea / China from gaining from the war and keep the war from spreading to involve a European Soviet threat. When it became clear MacArthur was pursuing his own theatre strategy of widening the war, and publicly criticizing Washington, Truman fired him.

Once MacArthur was dismissed, he was replaced by General Matthew Ridgeway who used rolling artillery followed by infantry advances to great effect in rolling back Chinese gains. But Ridgeway's objectives were different than MacArthur's. Ridgeway never approached the Chinese boarder as MacArthur had done. Ridgeway did cross the 38th parallel but only modestly to capture and hold better defensive lines. After that he fought a defensive war denying the communists any territorial gains from their initial invasion of south Korea, but also denying UN forces the ability to unify the Korea's.

@Argyll 1) Your troop strength number is dubious. Granted, the Chinese troop count is hard to confirm and 300k, your 180k number comes from a line on wikipedia page that has no citation. Here, a similarly unreferenced source, although it is superior in that the source accounts for the names of the units involved, puts the force ratio at 370k vs 723k;

The 300k number is not a controversial one. Here are a few other sources.

• Army Heritage Org Unknown to MacArthur, some 300,000 well-camouflaged Chinese soldiers had slipped into North Korea undetected and were waiting to spring their trap.
• CIA.gov CIA reporting from Tokyo, based on information obtained from a former Chinese Nationalist officer sent into Manchuria to contact former colleagues now in the PLA, stated that the PLA had over 300,000 troops in the border area. And, on 15 October, a CIA-led irregular ROK force operating on the west coast near the Yalu river reported that Chinese troops were moving into Korea.
• Smithsonian Institute In November 1950, 300,000 Chinese routed U.N. troops, forcing a retreat.
• The 300,000-man Chinese offensive caught the U.N. forces off guard
• October 1950 troops from the Chinese People’s Liberation Army began to cross the border to assist their North Korean ally. Their numbers grew to around 300,000 by early November
• On 6 November the FEC continued to list the total of Chinese troops in theater as only 34,500, whereas in reality over 300,000 CCF soldiers organized into thirty divisions had already moved into Korea.
• Part 1: How 300,000 Chinese Troops Snuck into Korea and Kicked the Ass of the U.S. Armed Forces, RW

your 180k number comes from a line on wikipedia page that has no citation

I have two sources for that number, again not controversial.

CIA: The Chinese Intervention in Korea, 1950
At the end of October, two more armies crossed to confront Eighth Army, for a total of 180,000 CCF.

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The Outbreak, 27 June-15 September 1950
On 1 September the United Nations Command had a strength of 180,000 in Korea:.... page 22

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@Argyll "odds" is not a mere numerical comparison, notwithstanding that I have not seen a source putting PVA + KPA at numerical advantage at the beginning phase of the war, MacArthur's command had total air supremacy and total naval supremacy. Its material advantage included numerous ground, air, and sea transport; it had firepower advantage in available artillery and close air support planes; having armor vs no armor. Yet that's still not the end of it, although hard to trace their origins, Chinese articles would describe PVA units lacking in mortars and had to rely on ineffective grenades.

Actually Oct Nov of 1950 was not the beginning phase of the Korean War. Just the beginning of china's formal involvement. The beginning of the war was North Korea's invasion of South korea in June of 1950, again when UN forces were at a disadvantage. It's true the UN had air superiority in June-Oct 1950 over the North Korean Airforce. However by Nov 1950 they were fighting the Chinese Airforce and by April they were fighting the Soviet Airforce. Likewise the Mig-15(620 mph) had about a 100 mile per hour advantage over the American F-80C(top speed of 502 mph) Shooting Star and the US didn't have as many Shooting Star's. Which is why the US had to use old WWII p-51 mustangs in a jet war.

@Argyll Truman and other higher authorities in the US government chose to rely on MacArthur's perspective in evaluating the Chinese/Soviet risk in terms of intervention.

Like I said Truman and the DC Policy makers took a gamble and China called them on it. I don't hold MacArthur responsible for their decisions, regardless of how they informed themselves.

@Argyll It is one thing to argue for the inadequacy in Truman's foreign policies. It is entirely something else -- and categorically false -- to say that "politicians" directed MacArthur to a gamble or "politicians" had determined to ignore the risk of Chinese/Soviet intervention

I never called Truman's foreign policy inadequate nor that MacArthur was "directed" to gamble. I said Truman's foreign policy was based on global concerns and he gambled in letting MacArthur approach the Chinese boarder..

As for why MacArthur was dismissed I don't have an issue with what you wrote.. I just don't think it disagrees with what I wrote.

Contrary to any claims by MacArthur, far and away the best defensive line anywhere on the Korean Peninsula is the east-west mountain range from Anju to Hamnung as shown here on an excerpt from the West Point Military Atlas.

In either the northern or southern version marked, it includes the vast bulk of North Korea's coastal plain, with its corresponding population concentrations. Its western flank runs to the sea along the southern branch of the Taeryong River to Anju.

Rather than assume a defensive posture following the Chinese counter attack, abandoning his indefensible and remote beachheads in North-western Korea, MacArthur insists on doubling down with an all-or-nothing strategy and gets thrown back to the 38th parallel - a far less appealing defensive line than the one he refused to adopt a few weeks earlier.

MacArthur, throughout this tenure as commander during the first months of this conflict, seems to have made a habit of ignoring principles clearly enunciated by Clausewitz that military commanders are to be subservient to the civil authorities that set policy:

the political view is the object, War is the means, and the means must always include the object in our conception.