Answer:
The US tried to remain neutral when WWI broke out, it finally joined the war on April 6th 1917, due to the submarine warfare. Germany had already sunk several merchant ships.
Step-by-step explanation:
The US initially contributed to the war by supplying row material and money, over 4000000 US military personnel were mobilized; 110000 Americans died during WWI, of which 43,000 lost their lives in the influenza pandemic.
The US saved Englands and other allies from bankruptcy by joining the war. Britain and its allies used to buy supplies from the US for more that 75 billion dollar per week. The US also reinforced the allies' strength of the naval blockade of Germany.