Answer:
Germany started sinking American merchant ships in the North Atlantic
Step-by-step explanation:
The First World War started in 1914. At first, President Woodrow Wilson ensured all the countries that were involved in the war that United States will remain neutral.
But, after Germany started sinking passengers ships that belong to Great Britain (which is United States' closest Ally), US citizens started to pressured the government to enter the war joining the Alliance. This is the reason why USA entered the war in 1917.
As soon as Germany realized US' involvement, they started to sent their submarines to the North Atlantic. They attacked the merchant ships that passed through that territory in order to Hurt US' economy.