This is a very interesting question.  I think that one could argue that American isolationism's fear of getting involved in a protracted conflict actually helped to cause one.  The reality was that the 1930s in Europe was a decade dominated by the singular, charismatic leader.  Democracies did not stand a chance given the economic, social, and psychological fragmentation that gripped Europe in the wake of the First World War.  Leaders popped up all over Europe and sought to build consensus and dominate through the overtaking of nations and the expansionist model of nation building.  American isolationism ended up feeding into this growing advance.  For example, Hitler's desires to appropriate more land and expand Germany through military aggression were goals that were emboldened with American isolationism.  The tenet of America not seeking involvement amounted to a silent nod to aggression.  In this, war became a distinct and more definable reality.

There is no way to conclusively prove that US isolationism and neutrality actually contributed to the spread of the war.  However, you can argue that the US could have prevented the spread of war if it had made clear to Germany and Japan that it would fight them if they took any aggressive actions towards their neighbors.

The logic behind this is that a US ultimatum like this would have scared Germany and Japan and prevented them from taking the actions that led to war.  For example, if the US had credibly threatened war with Japan if Japan moved into China, you can argue that Japan would not have made this move and WWII might have been less likely.

However, this seems unlikely since the US was hardly in position to attack Germany or Japan.  That makes it very unlikely that either country would have been scared by US threats.  In order for this to have worked, the US would have to have built a huge military machine even in peace time so as to scare the Axis.