What determined the timing of the U.S. entry into the Great War?

The States were not eager to get involved in the Great
War; but, of course, the Americans having a commun story with the English - and  "
separated " by the same language, were very sympathic to The United Kingdom against the
Tweite Reich.

By 7 May 1915, a German Untersee Boot hit the
Lusitania, an USA registered vessel. It sank with about 1,200 people a
board.

Now, this ship was transporting ammunition to one of
the countries involved in the war; and the Germans were aware of it, thanks to
informations out of the USA.

This way the Germans pretended
to have the right to blow up ( down ) the ship; that violated America's
neutrality.

President Wilson of the States has always been
avoiding to get involved, but Germany lost all sympathy in the
States.

By 1917 is launched a huge U-boat offensive in the
Atlantic. The Czar is in jeopardise weakening the east front. Germany stries to convince
Mexico to get on its side.

Too much, Wilson said ! And he
sent a message to the Senate.

By 6 April 1917 the USA
declares war on Germany.