Question: From a British perspective in both WW1 and WW2 the USA have entered the war at a later stage and supported the allies.

The decision for the US to enter the war has often been portrayed to be based on particular acts of aggression against them. The sinking of the Lusitania and the attack on Pearl Harbour.

Was there ever any doubt which side the USA would join? Is there evidence to suggest that these triggers decide which side the USA would join as well as the level of their involvement?

Answer: 
I know the answer for the Second World War. The political parties and organizations were all pro Allies except three notable groups:

[German American Bund](http://en.wikipedia.org/wiki/German_American_Bund)  

This was a Nazi organization supported by Germans in the USA. It evolved simultaneously with the NSDAP, but never got enough support to have any notable power. When the war broke out, the organization became unfavored in many ways: there was a tax investigation, and later they refused to be drafted so the members got fined for $10000 and 5 years in jail. By the mid-late war the leaders were in jail or had their citizenship revoked, so the whole organization stopped working.

[Silver Legion of America](http://en.wikipedia.org/wiki/Silver_Legion_of_America)  

This was a fascist party in USA which ran in the elections of 1936, but never really got more than 10000 members. Right after Pearl Harbor, the local police occupied the HQ of the Silver Legion and the membership of the party left as the war went on.

And one more notable and popular organization is the [America First Committee](http://en.wikipedia.org/wiki/America_First_Committee), which boasted famous aviator Charles Lindbergh as a spokesman. They wouldn't enter into the war on the Axis side, but they strongly argued against entering on any side, even the Allies'. I mention them, because if their points had gotten approval — such as opposing the Lend Lease Agreement, or being neutral — it could have seriously affected the outcome of the Second World War.

**So for the Second World War: there was no doubt that the USA was a pro-Allies power, as these organizations were a small minority when Pearl Harbor occurred, and most of the people supported the Allies' cause.**
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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/M3_Lee), was not in production until August 1941. The ubiquitous [M4 Sherman](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/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](https://en.wikipedia.org/wiki/German_declaration_of_war_against_the_United_States_%281941%29). 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.