In August, 1914, Europe erupted in warfare For nearly three years, President Woodrow Wilson kept the United States out of the war, even in the face of actions that could have justified US entry.
American lives were lost on the high seas when German submarines sank the British civilian passenger ship Lusitania without warning and without stopping the ship and boarding it to determine if the cargo included contraband or offering passengers the opportunity to escape in life boats. These were well understood measures in maritime law intended to protect civilian life. To sink the ship without warning violated international law and custom.
Germany justified its actions because, since the last maritime war in the early 19th century, Marconi's invention of the radio made the traditional procedures too hazardous for warships to follow. The United States protested and Germany relented to a certain extent.
But Germany was becoming desperate. Enough so that they sent a telegram to Mexico offering to return territory taken by the US in 1846 to Mexico if they would join the war if the United States entered on the British side. That Zimmerman telegram alone could have been enough to justify US entry. But Wilson held off.
Then, in January 1917 a desperate Germany renewed unrestricted submarine warfare, sinking merchant ships on the high seas without warning no matter what flag they were flying. The goal was to starve England and France to the negotiating table. They expected this would cause the United States to enter the war, but expected they could defeat the allies before the US could recruit, train, equip and transfer to Europe a force sufficient to tip the balance against them.
It was a bad bet.
Last week PBS broadcast a three episode series showing the US entry into World War I: http://www.pbs.org/wgbh/americanexperience/films/great-war/
If you missed the series, I recommend you seek it out and watch it.
Consequences of that decision produced powerful effects whose ripples are with us to the present time.