Describe the German actions which led the United States to enter the war.

I assume this question refers to World War I, since it was Japanese actions that led the United States into the Second World War. The United States reluctantly entered World War I because the German government reinstitute a policy of unrestricted submarine warfare in 1917. Under pressure from a British blockade of its ports, Germany retaliated by sinking merchant ships headed for Great Britain. Eager to avoid conflict with the United States, many of whose ships were placed in harm's way, the German government pledged in 1916 to refrain from sinking merchant or other civilian ships without fair warning. When, in 1917, the Germans decided to renege on this promise, and began attacking American ships, war seemed imminent. Matters came to a head when a German telegram proposing an alliance with Mexico was translated and made public in the United States. This so-called "Zimmerman" telegram (after its sender) proved to be the final straw. In April of 1917, just one month after President Woodrow Wilson was inaugurated after a reelection campaign in which he trumpeted the fact that he "kept us out of war," the United States declared war on Germany.