What caused world war one and why did the us enter the war?

U.S. Entry into World War I, 1917. ... Wilson cited Germany's violation of its pledge to suspend unrestricted submarine warfare in the North Atlantic and the Mediterranean, as well as its attempts to entice Mexico into an alliance against the United States, as his reasons for declaring war.

Explanation:

abraham lincoln  (february 12, 1809  – april 15, 1865) was an american statesman, politician, and lawyer who served as the 16th  president of the united states  from 1861 until  his assassination  in april 1865. lincoln led the nation through the  american civil war, its bloodiest war and its greatest moral, constitutional, and political crisis.[2][3]  he preserved the  union,  abolished slavery, strengthened the  federal government, and modernized  the u.s. economy.

a. by accepting the election of president abraham lincoln in 1860 and convincing other southern states to do the same