The main reason for the American entry into World War I was the German use of submarine warfare (1917), violating the United State right to freedom of the seas. German was suffering from near- starvation due to a lack of supplies and food. German military leaders feared they would lose the war unless they could defeat Britain and France quickly. They therefore took a risk by announcing they would they would sink any ship in the blockaded areas. This was a clear violation of the American principle of freedom of the seas that neutral parties have the right to ship non-war goods to nations at war. When German followed up its threat by ordering its submarines to sink three unarmed American merchant vessels, Wilson asked congress for a declaration of war against German.
This is often referred to as unrestricted submarine warfare which Germans thought theirs subs were allowed to sink any other ships without warning. After destroying several American ships that were supplying Great Britain, President Wilson warned Germany. Even after the warning because Germany was running out of war supplies they had continued to go about this method in hopes to remove the naval blockage. This eventually angered President Wilson which soon led to America entering World War I.
As technology and weapons began to advance, the Arms Race became a crucial part to winning the war. U Boats or submarines were one of the many new pieces of equipment which aided many countries with forces in the sea. Other advancements such as gas masks and machine guns also gave a few countries a leg up in the war. Gas masks were used to protect soldiers from chemical weapons such as mustard gas which killed many people who breathed it in.