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 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.