Who was the last to join the allied fighting force? germany france great britain united states

The right answer is  united states. Given that the War was not specified let us take a look on both wars. In both wars USA was the last to join the allied fighting force, because of its neutrality policies and its geographic neutrality.

WWI:  The USA came into war, on the side of the Triple Entente (or Allies), in April 1917, after Germany had resumed its unrestricted submarine warfare. By this time, France and Britain were already at war.

WWII: The USA declare war on Japan on  December 8, 1941 after the attack of Pearl Harbor. Putting an end in the American neutrality again. And again, France and Britain were already at war.

u

explanation:

a historian researching the immigration policies of the early 1900s would expect a source written today,rather than in 1910 to :

( a ) be more interested in objective data and statistics.