Franklin Roosevelt was a student of history, and knew that a two-front war was simply bad strategy.  You divide your manpower and your resources in two directions and are therefore half as strong or effective on each front.  This was the German experience during World War I, and such a strategy, among other things, led to their defeat.  Hitler repeated the mistake in World War II.  FDR was determined not to do the same.

The plan was to concentrate 80% of our resources to the European front and 20% in the Pacific to hold the Japanese at bay.  What happened in actuality was that our military and navy grew so massive that we were able to prosecute the war on both fronts successfully, and very few soldiers had to be shipped from Europe to the Pacific once the war there ended.  There are very few countries on Earth that could fight a two front war successfully.

No one really wants to fight a war on two fronts.  If you have to fight on two fronts at once, your power is badly diluted.  You have to send soldiers and weapons and such to both fronts instead of being able to concentrate all your resources on one front.

In the case of WWII, FDR agreed with Winston Churchill that the Allies should concentrate mainly  on Europe first.  However, it would not have been possible to simply let Japan do what it wanted in the Pacific and that meant that the US was going to have to fight on two fronts.