Question

what was the major cause for the entry of the U.S.into world war one

Answer 1

Germany's resumption of submarine attacks on passenger and merchant ships in 1917 became the primary motivation for Wilson's decision to lead the U.S into WWI. Following the sinking of unarmed French boat the Sussex.

Answer 2

The major cause for the entry of the United States into World War I was  the event known as the sinking of the Lusitania ship, which started an anti german feeling within the american public. This was followed by the sinking of several American ships,  which led the Congress to declare war on Germany on April 6, 1917.