Updated!

In 1940 and 1941, the U.S. Navy and Coast Guard were engaged in what at least one historian has called an "undeclared war" in the Atlantic. American warships chased German submarines, tipped off British warships and convoys, and even fired on the Germans!

A timeline of the relevant events:

• 2 September, 1940: In the Destroyers for Bases Agreement, FDR gives the British 50 WWI-era destroyers in exchange for long-term lease on British bases in the Caribbean.
• September 1940: The British Tizard Mission arrives in the United States with radar, sonar, electronics, and other technology. These innovations are refined and utilized by the U.S. armed forces in the coming years.
• 29 December 1940: FDR gives the "Arsenal of Democracy" speech and begins to ramp up American production to meet Allied and American demand.
• 11 March, 1941: The Lend-Lease Act is enacted, giving the British "loans" (effectively gifts) of raw materials, aircraft, tanks, and other military equipment. Lend-Lease to the British will continue until nearly the end of the war.
• 4 April, 1941, American shipyards become available to British and other Allied vessels for repair and refitting, an act that was a clear violation of American neutrality. Five days later the British battleship, HMS Malaya arrives n New York for repairs. The bill is paid for by Lend-Lease.
• 10 April, 1941: The destroyer USS Niblack drops depth charges on a U-boat as it rescues the crew of a torpedoed Dutch freighter off the coast of Greenland. This is the first combat action taken by an American naval vessel against the Germans.
• 7 July, 1941: U.S. Marines land on Iceland and establish a base there. Roosevelt ordered a war zone around Iceland and tells his military chiefs that “the approach of any Axis force within 50 miles of Iceland was to be deemed conclusive evidence of hostile intention and therefore would justify an attack by the armed forces of the United States.”
• 9 August, 1941: FDR agrees at the Newfoundland to provide American destroyers as escorts for the Canada-Iceland leg of British convoys across the Atlantic. American, Britishb and Commonwealth forces share some information, like U-Boat sightingsb with each other.
• 4 September, 1941: In the Greer Incident, the destroyer USS Greer (DD-145) is fired on by U-652 while patrolling in the Atlantic.
• 4-5 September: U.S. Navy begins air and surface patrols of the Atlantic coast and the Caribbean. FDR declares this to be a "Neutrality Patrol."
• 11 September 1941: FDR issues the "Shoot on Sight" order, which declares that any German submarines sighted in waters "which American deems vital to its defense" will be attacked without warning.
• 16 September 1941: The first Atlantic convoy with a U.S. Navy escort heads to sea.
• 17 October 1941: USS Kearny (DD-432) is torpedoed while escorting Convoy SC 48. Several sailors die.
• 31 October, 1941: While escorting a British convoy as part of the Neutrality Patrol, the USS Reuben James (DD-245) is torpedoed and sunk by U-552 while escorting convoy HX 156 in the North Atlantic. 100 American sailors die. The attack takes place five weeks before the attack on Pearl Harbor.
• 11 November, 1941: U.S. Navy captures the German blockade runner Odenwald off the coast of Brazil with the cruiser USS Omaha and the destroyer USS Somers.
• 11 December 1941: Germany declares war on the United States.
• 1 March 1942: A PBO-1 Hudson of VP-82 flown by Ensign William Tepuni sinks U-656 with all hands. It's the first American U-Boat kill of the war.