Although having helped since 1915 the Entente powers with goods and armaments, the US remained neutral until April 1917; the German unlimited submarine warfare, which caused great economic damages, forced finally the government to enter the war. In the first period, with the exception of the fleet, the military contribution of the US forces was limited; since the early months of 1918 hundreds of thousand of soldiers were deployed on the Western front, changing the unstable equilibrium on the field. At the other side, the Central Powers, illuded by the conclusion of the war on the East, were unable to deploy fresh troops. But what was the American contribution to the war? And what the consequences of this participation at the economic level?