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?