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.**
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.