Because WW2 had a major breakdown during it's first years of war, alliances and treaties were one of the most important things during it's time. An alliance is when two or more states create an agreement to benefit one another, and treaties are where you rationalize the agreement. Because of this, it settled out many countries during the war. For example, the major allies in WW2 are U.S.A, Britain, France, and Russia. (Of course this isn't the full list, but these are the major ones.)
For this, it was a major role to enlist all those who were in the war.