America joined World War I in April, 1917. With tensions building since 1915, President Wilson finally determined that American interests and lives, increasingly threatened by unrestricted submarine warfare on the part of Germans, required military action. War was declared on Germany, and within a few days America joined its allies in the struggle. However, it would be several months before sufficient troop strength could be mobilized. By March of 1918, only 318,000 Americans had arrived in France, and they were severely overmatched at first.