Ultra intercepts of German radio signals alert Royal Navy to a convoy of 4 German troopships & 1 Italian ammunition ship carrying 15th Panzer Division troops and light vehicles (but no tanks) from Naples to Tripoli. British destroyers HMS Jervis, Janus, Nubian & Mohawk lie in wait after dark, unnoticed by the escort of 3 Italian destroyers which have no radar, as the convoy moves slowly through shallow waters near the Kerkennah Islands (East coast of Tunisia). British destroyers close to within 1 mile and sink all 5 transports and the Italian destroyers Luca Tarigo, Baleno and Lampo (1800 German troops and Italian sailors killed, 1200 survivors rescued by Italian warships and hospital ships). As Italian destroyer Luca Tarigo sinks, she fires 2 torpedoes which cripple HMS Mohawk (43 killed). HMS Mohawk is scuttled by HMS Jervis but Italian divers recover secret Royal Navy documents including maps of the defenses at Alexandria.
Determined to crack the perimeter at Tobruk, Rommel observes in person as Italian Trento Division is sent forward in late afternoon. They are shelled as they assemble for the attack, which drives off their tanks, and the infantry are pinned down with machinegun fire. Australian Bren gun carriers are sent out and round up the entire 1st Battalion, 62nd Regiment (800 men including 25 officers). This is the second time in 2 days that Italian troops have dispersed when shelled and Rommel is losing faith in his ally.
In Greece, while Allied troops retire towards Thermopylae, the defensive line in the North begins to crumble. German tanks and infantry attack the narrow Platamon pass between Mount Olympus and the Aegean Sea, forcing New Zealand troops to withdraw across the river in the steep Tempe Gorge (using a flat-bottomed ferry pulled by hand along ropes). Realizing that German penetration along the coast will unhinge the Allied withdrawal, New Zealanders are ordered to sink the ferry and hold the Gorge “until 19th April even if it means extinction”. German 6th Mountain Division also attacks across Mount Olympus using goat paths but takes heavy casualties.
Yugoslavs try to sign an armistice but their emissary does not have sufficient authority to sign the surrender and is sent to back Belgrade with a draft agreement. In Zagreb, Croatian fascist leader Ante Pavelić assumes power in the Independent State of Croatia.
At the end of her cruise and heading for port at Bordeaux, France, German armed merchant cruiser Thor stops Swedish steamer Sir Ernest Cassell with 2 warning shots, takes her crew on board and sinks her with demolition charges (500 miles Southwest of the Azores).
Overnight, 681 Luftwaffe aircraft launch a massive bombing attack London.