Was there ever any doubt which side the USA would join?

Alfred Thayer Mahan's doctrine pretty much dictated that the United States will be fighting alongside the British, who were the other naval superpower, rather than Germany which was a land power.

Japan is an interesting case, while at the start of WWII it demolished the British navy, raw racism precluded any possibility of alliance. Recall Japanese immigrants were subject to all kinds of exclusions in the United States at the time, and the implementation of internment.

In any case, the United States wasn't about to share the Pacific anyway.

No, the side we would take if we were to enter both wars was undisputed. The only problems was the absence of public unity about going to war. This was a primary concern when America entered the war on April 6, 1917. In Washington, unwavering public support was considered to be crucial to the entire wartime effort. That is why on April 13, 1917, President Wilson created the Committee on Public Information (CPI) to promote the war domestically while publicizing American war aims abroad. Walter Lipmann and Bernays were crucial on this front.

accessible article http://www.propagandacritic.com/articles/ww1.cpi.html. Lipmann's Public Opinion excellent source to see the philosophy behind it.

Bottom line up front. Yes their was a doubt. Germany had a vocal advocacy in the US as large or larger than did the UK. The UK had historical ties and FDR. Ultimately though neither was enough to move Isolationist America. America did not declare war on Germany or Japan first. They declared war on us and only then did we reciprocate. And some folks in the US are still hot about that even today.. (aka Pat Buchanan ).

Longer but the same Answer The United States had been a strictly isolationist country devoid of all foreign security treaties and neutral in all European wars from 1789 (George Washington's farewell Address) - April 1917 until we committed for the Allies in WWI.

George Washington Farwell Address ( See Foreign Relations & Trade ) https://en.wikipedia.org/wiki/George_Washington%27s_Farewell_Address#Foreign_relations_and_free_trade

United States non-interventionism, (SEE) No entangling alliances (19th century), and Non-interventionism before entering World War II https://en.wikipedia.org/wiki/United_States_non-interventionism#No_entangling_alliances_.2819th_century.29

America entered WWI for more pragmatic reasons than the sinking of the Lusitania or Zimmermann Note. Our banking system lead by JP Morgan was heavily invested in the success of the Allies. US banks funded the Allies war effort. JP Morgan organized a syndicate of about 2200 American banks and floated loans exceeding $500 million to the Allies. That was big money in 1912. The British sold off their holdings of American securities and by late 1916 were dependent on unsecured loans for further purchases. If the United States had not entered into that war and the Allies had lost our economy would have taken a significant hit.

https://en.wikipedia.org/wiki/J._P._Morgan_Jr.

The US had pragmatic reasons for involving ourselves in that war. The US was not afraid of Mexico in 1917, we were at war with troops on the ground in Mexico in 1917, we had invaded Mexico in response to Poncho Via's raids. We knew they posed no risk to our security. https://en.wikipedia.org/wiki/Pancho_Villa_Expedition#Aftermath

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As for WWII, there was a strong pro German faction in the United States lead by World Famous Industrialist Henry Ford, Democratic Presidential Candidate Joe Kennedy, and American Aviation Hero Charles Lindberg. Consisting of organizations like the American First Committee, The German American Bund, and Friends of New Germany. Nazi Germany's interests were well represented and supported in a fractured pre WWII America. The Latter two, The German American Bund and Friends of New Germany were basically clones of the Germany's Nazi Party operating openly in the United States.

https://www.salon.com/2015/01/02/fdr_outsmarts_them_all_henry_ford_joseph_kennedy_charles_lindbergh_and_the_american_entrance_into_world_war_ii/

German American Bund https://en.wikipedia.org/wiki/German_American_Bund

Friends of New Germany https://en.wikipedia.org/wiki/Friends_of_New_Germany

Ford being a famous respected industrialist and infamous anti semite, who had long ago captured the worlds imagination. Kennedy being one of the wealthiest and successful men in the United States would latter achieve infamy and political death when he favorable compared the Nazi Party to the American Democratic party (of which he was a leader). Lindberg being perhaps the most famous American ever, and one of the most respected who speeches still can be listened too and found persuasive in why the US should stay out of that war. The pro German, and anti war FDR critics were outspoken and influential. Even stronger though than the pro German faction was the symbiotic Isolationist faction. Not to be confused with pacifism, America's brand of Isolationism as first outlined by Washington in 1789 proposed the US not getting involved in European wars, but did not suggest America should refrain from conducting her own wars to protect her interests. These two beliefs (1)Isolationist from European entanglements, and (2) those who favored Nazi Germany; worked together because for Germany's purposes an truly isolationist United States was sufficient to its needs.

The influential "American First Committee" characterized by anti-semitic and pro-fascist rhetoric. Started on September 4, 1940, it was dissolved on December 10, 1941, three days after the attack on Pearl Harbor had brought the war to America. Membership peaked at 800,000 paying members in 450 chapters. It was one of the largest anti-war organizations in American history. Membership included future US Presidents, future Supreme Court Justices, and other future famous government leaders.

https://en.wikipedia.org/wiki/America_First_Committee

Even though we were a wealthy, and populous country we did not have a military. The United States military on the outset of WWII was about the same size as Belgium's or Portugal's. A historically isolationist country like the US had no need for a significant standing army.

http://www.politifact.com/texas/statements/2014/jun/13/ken-paxton/us-army-was-smaller-army-portugal-world-war-ii/

(*) The United States has been the world's largest national economy in terms of GDP since at least the 1920s https://en.wikipedia.org/wiki/Economy_of_the_United_States#cite_ref-Digital_History_41-1

And Isolationism/Non Interventionism (I don't differentiate) dominated American politics. Perhaps most Americans were sympathetic to Britain and saw Germany as the clear aggressor, But they did not see it as any of America's business. America's brand of Isolationism, did not advantage either the aggressor or victim. We saw ourselves as small fish, easily devoured when the big fish quarreled. Also, Lindberg and Joe Kennedy painted admirable images of Hitler raising the German people out of despair and poverty and making them productive again. America was against the punitive actions taken by the victors of WWI against Germany. Americans believed these actions had impoverish Germany, and Americans had empathy for the German's economic hardships. That Hitler and the Nazi's were changing this unfair impoverishment in Germany was a good thing right! This argument played on US sympathies. Isolationism was well supported by America's political leadership, reflecting the support it had with the population.

Source (http://www.ushistory.org/us/17d.asp). See Paragraph 8. Source (http://www.u-s-history.com/pages/h1601.html)

America was by no means a militarily significant power on the level of Britain, France or Germany at the time. Remember, Belgium was overrun by the Germans in an afternoon. The US military which trained with stove pipes and broom sticks instead of mortars and machine guns as late as 1941, were seen as no match for even the weakest of those big three. Perhaps a match for Belgium, by contemporary wisdom.

(*) "as late as 1941, U.S infantry mortar squads used stove pipes as simulated mortars, Machine-guns were improvised from broom sticks." http://www.dtic.mil/dtic/tr/fulltext/u2/645160.pdf

Maybe not as well equipped as Belgium's. Isolationists made note that Germany had just defeated the strongest army in the world in France (contemporary wisdom 1938), and had the second strongest Army the UK on the ropes.. what were we going to do; especially all the way over in Europe. In the history of modern day Europe no foreign military had ever staged a successful invasion of the continent, and the US barely had a military!! This argument given where the US was in 1938 on the outset of WWII, is even today a very logical, historically supported, and thus persuasive argument (give it a listen bellow) even though in hind site it proved to be entirely wrong.

The America First Committee Charles Lindbergh Speech September 11, 1941 Des Moines https://www.youtube.com/watch?v=K_F48oaOskI

Franklin Delano Roosevelt had just won reelection in 1940, famously stating "Your boys are not going to be sent into any foreign wars".
This statement came due to a political necessity, even FDR a man running for an unprecedented 3rd term as president did not have the political legs to confront directly the non interventionists.

10/40/1940 Campaign Address in Boston Ma.
http://www.presidency.ucsb.edu/ws/?pid=15887

Of coarse today we know Roosevelt was already actively involved in impeachable offenses to provide Britain with significant material aid. Roosevelt was against Germany, and was working behind the scenes to keep the UK in the war, to prepare the US military, to change US public opinion, and to get us into the war as quickly as he could.

Roosevelt made the statement in Boston because he would not have been elected had he not. Roosevelt was a consummate politician who knew the country would not support another foreign war and his hands were tied legally from supporting Britain.

The Neutrality Act of 1939 allowed belligerents to purchase war materiel from the United States, but only on a “cash and carry” basis. Britain had little cash. The Johnson Act of 1934 also prohibited the extension of credit to countries that had not repaid U.S. loans made to them during World War I—which included Great Britain. The US military believed Great Britain would surrender after the surrender of France, and any weapons sent to her would ultimately end up in German hands. On this basis they opposed sending weapons to the UK. President Roosevelt had to develop an initiative that was consistent with the legal prohibition against the granting of credit, satisfactory to military leadership, and acceptable to an American public that generally resisted involving the United States in any foreign(European) conflict.

(Sept 1940) Just a month before Roosevelt made his famous declaration in Boston to stay out of the war, He had begun transferring significant aid to Britain illegally. Britain could not afford to legally purchase the munitions she required so Roosevelt got creative with non monetary exchanges (base leases in North America). Because these exchanges were non monetary they were in violation of US law. In December 1940, Churchill warned Roosevelt that the British were no longer able to make any payments for supplies. So Roosevelt went even further. On December 17, 1940 Roosevelt proposed lend lease of supplies. Making the analogy if your neighbors house is on fire and he wants to borrow your hose, of coarse you are going to lend him your hose, and after the fire is out, you don't want his money; you want your hose back. Roosevelt ear marked 1 billion dollars in war aid for Great Britain and Russia along lend lease.

Just 4 months before Pearl Harbor Congress believing disarmament to be the surest way to ensure America's neutrality had nearly disbanded the US Military. The measure being defeated in the house by a single vote in August 1941.

Source (http://www.nytimes.com/1991/09/01/opinion/l-the-vote-that-saved-the-army-in-the-days-after-pearl-harbor-028191.html) See: The Vote(august 1941)That Saved the Army in the Days After Pearl Harbor -

That's the environment the United States and FDR had to deal with, and how strong the Isolationists were.... And of coarse ultimately the United States did not decide to over come it's isolationism tendencies, or even it's pro German tendencies. The United States was attacked, and her sons and daughters were murdered in a surprise attack, infuriating the nation. Then Japan and Germany declared war on the United States. There was no choice for war made in the United States to enter WWII. The "choice" to enter the war on behalf of the allies was not made in Washington DC. It was made in Tokyo and Berlin. Amazingly Germany had been bending over backwards to not provoke America into the war on Hitlers direction. He was aware of the delicate dance FDR was stepping. After Japan declared war, in arguable the stupidest move of the war. Germany followed suit and likewise declared war on the US in perhaps the second stupidest move.

Dec 7th, 1941 -- Japan declares war on the United States
https://en.wikipedia.org/wiki/Japanese_declaration_of_war_on_the_United_States_and_the_British_Empire

December 8, 1941, the United States Congress declared war on Japan https://en.wikipedia.org/wiki/United_States_declaration_of_war_on_Japan

Dec 11, 1941 -- Germany Declares War on the United States http://www.history.com/this-day-in-history/germany-declares-war-on-the-united-states

On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the US was still officially neutral during World War II. The decision to declare war was made by Adolf Hitler, apparently offhand, almost without consultation. Later that day, the United States declared war on Germany. https://en.wikipedia.org/wiki/German_declaration_of_war_against_the_United_States_(1941)

Although Hitler made a few stupid moves during the war, some of which compete on the scale with declaring war on the US while his failure to do so arguable could have kept the US out of the European theatre at least for a while. Hitler's pact with Japan did not require Germany to declare war on the US if Japan was the aggressor.. Thus Hitler literally forced FDR's hand, to FDR and I imagine Churchill's everlasting gratitude. This solved all of the US political problems and allowed the US to come into the war committed and unified even though just days before the war she was far from that.

Uniting a badly divided America, and ensuring she would be fully committed to the Allies.

Although I agree broadly with CsBalazs, we should not overlook the fact that there was a rather strong pro-Nazi sentiment among the upper classes, and especially the academic elite, in the US. For example: Martin Sprengling, director of the Oriental Institute in Chicago ; Walter Nitze, head of the Romance faculty in Chicago ; George Kingsley Zipf, teacher of German at Harvard and inventor of “Zipf’s law” ; all of them were self-proclaimed Nazis, but no one ever seems to talk about this. By the way, pro-Nazi thinking was also very widespread among the aristocracy and upper-middle classes in Britain.

During WW1, the U-Boat attacks DID have a significant effect on turning American opinion against Imperial Germany. Furthermore, it is unlikely that the US would have entered the war without the sinking of the Lusitania or another similar provocative event. It is often forgotten in this day and age that the second largest ethnic heritage group in the United States is German, outnumbered only by the English. The rebranding and suppression of German culture during WW1 was significant, far beyond anything before or since in this country. Those who laugh/sneer at "Freedom Fries" probably think nothing of eating cream cheese, or as it used to be known prior to WW1, schmeerkase. Stop and think about it for a moment. Why does the second largest heritage group in America has such a small distinct cultural footprint today?

With regards to WW2, US entry without provocation was much more likely. Contrary to what others have said, the US was not "grossly unprepared" for the war. During WW2, the US Navy took delivery of 9 brand spankin' new battleships, 2 battlecruisers, and 2 dozen fleet carriers. ALL of the battleships were under construction before the war started, as were a third of the carriers. The expansion of the Army started in 1940. Almost every major weapon system used by the US during WW2 was designed before the war, and either in development or production when the war started. The upshot is "grossly unprepared" is a gross overstatement. Unprepared? Yes, certainly tactically on that Sunday morning in December. Preparing? Definitely. The US was rearming with a speed and breadth that is astounding.

During WWII, there was less doubt about which side the US would join. War with Japan had been anticipated for years and a strategy of waging war adopted in the 20s called War Plan Orange. With Japan's expansionist policies, and the US' territories in the pacific (Guam, Philippines, Wake) it was more a matter of when than if.

The US considered Germany less of a direct threat. Germany was not a great maritime power, seaborne invasion was not a threat. With no territorial interests in Europe, US had no direct stake in the war. There was a very strong isolationist movement in the US which Roosevelt had to carefully step around to supply the Allies. The more the US got involved in supplying arms to the Allies, the more they came into conflict with German U-boats, the more likely war became.

However, there is a great question of when the US would have entered WWII. It likely would not have been December 1941. The US military was grossly unprepared for war. In 1940 it was largely a reserve force of a few hundred thousand. Due to the delay in rearming and the chaos caused by rapid expansion, Army divisions were still being assembled, trained and brought up to strength. They were far from ready for the complicated logistical task of deploying overseas. The first viable, but rushed and flawed, US medium tank the M3 Lee, was not in production until August 1941. The ubiquitous M4 Sherman would not roll off production lines until 1942. The US Navy, while prepared to fight a conventional war in the pacific, was unprepared for defensive submarine warfare as shown in Operation Drumbeat.

The Attack On Pearl Harbor was a strategically decisive defeat for the Axis in many ways. It forced the US to enter the war early, removing the need to maintain a veneer of neutrality for its own citizens and ramping up production earlier than it would have otherwise. It over strained the Japanese military who were now attacking in every direction, had they waited they could have fought a more concentrated defensive war with preparations, or no war at all!

Second, and this is often overlooked, Pearl Harbor caused Germany to declare war on the US. It is often assumed it was the other way around, that the US automatically declared war on Germany, not the case. There was a great risk of having the US fight Japan and, wanting to put all its attention and resources into that war, not fight in Europe. Germany's declaration of war eliminated the possibility for the US to be knocked out of the war in Europe without firing a shot.