USS Greer (DD–145) was a Wickes-class destroyer in the United States Navy, the first ship named for Rear Admiral James A. Greer (1833–1904). In what became known as the "Greer incident," she became the first US Navy ship to fire on a German ship, three months before the United States officially entered World War II. The incident led President Franklin D. Roosevelt to issue what became known as his "shoot-on-sight" order. Roosevelt publicly confirmed the "shoot on sight" order on 11 September 1941, effectively declaring naval war against Germany and Italy in the Battle of the Atlantic.

On September 3, 1939, the British and French declarations of war on Germany initiated the Battle of the Atlantic. The United States Navy Chief of Naval Operations (CNO) established a combined air and ship patrol of the United States Atlantic coast, including the Caribbean, on 4 September, President Franklin D. Roosevelt declared the United States' neutrality on 5 September, and declared the naval patrol a Neutrality Patrol. Roosevelt's initiation of the Neutrality Patrol, which in fact also escorted British ships, as well as orders to U.S. Navy destroyers first to actively report U-boats, then "shoot on sight", meant American neutrality was honored more in the breach than observance.