Not the only purpose to be sure, however it was explicitly mentioned as a rationale for staying out of the European theater completely.

The idea was to allow Germany and Russia to slug it out, completely exhausting both countries.

See for example the views of Charles Lindbergh (search for Russia) : http://www.pbs.org/wgbh/amex/lindbergh/sfeature/fallen.html

America's strength is why the USD became the reserve currency of the world. Note that the Bretton Woods conference was held in 1944, before the war ended: http://en.wikipedia.org/wiki/Bretton_Woods_system