Why did the u. s. become involved in ww1?

In 1917, Germany, determined to win its war of attrition against the Allies, announced the resumption of unrestricted warfare in war-zone waters. Three days later, the United States broke diplomatic relations with Germany, and just hours after that the American liner Housatonic was sunk by a German U-boat. On February 22, Congress passed a $250 million arms appropriations bill intended to make the United States ready for war. In late March, Germany sunk four more U.S. merchant ships, and on April 2 President Wilson appeared before Congress and called for a declaration of war against Germany. Four days later, his request was granted.

What does this mean ? ?