On the Western Front... The Germans launch Operation Gelb, the offensive in the west. Army Group C (Leeb) holds the German frontier opposite the French Maginot Line while Army Group A (Rundstedt) makes the main attack through the Ardennes and Army Group B (Bock) makes a secondary advance through Belgium and Holland to draw the main British and French forces north. During the day, Army Group A strikes, with three armored corps in the lead, heading for Sedan, Montherme and Dinant. The advance is rapid and the little opposition, mostly French cavalry, is thrown aside. To the north, Army Group B carries out parachute landings deep inside Holland which do much to paralyze Dutch resistance, while German units cross the Maas River near Arnhem and the Belgian fort at Eben Emael is put out of action by a German airborne force which lands its gliders literally on top of it. The fort is meant to cover the crossings of the Albert Canal nearby and this is not achieved. The Luftwaffe gives powerful support. At the end of the day the German advance has gone almost exactly according to plan. Meanwhile, the Allied Plan D provides for the French 1st Army Group ( General Billotte), consisting of the British Expeditionary Force ( General Lord Gort) and the French 7th Army (General Giraud) to advance to the line of the Dyle River and the Meuse River above Namur, to be joined there by the Belgian forces and on the left to link with the Dutch. General Gamelin is the Allied Supreme Commander and General Georges commands the armies on the French Northeast Front. The Allies react quickly to the German attacks as soon as they hear of them from the Belgians. By the evening much of the Dyle line has been occupied but the troops find that there are no fortifications to compare with the positions they have prepared along the Franco-Belgian frontier during the Phony War period. Some of the reserve is therefore committed to strengthen the line. Some of the advance forces of French 7th Army make contact with the Germans in southern Holland and are roughly handled.

In Britain...Churchill visits the King and officially takes office as prime minister.

In Norway... British forces are sent south from Harstad to Mo-i-Rana to join the small units trying to delay the German advance to relieve the Narvik force. Some of these units are now engaged at Mosjoen.

In Iceland... British troops land on the island. They are the advance elements of a force which is to set up a destroyer and scout-plane base to help in the convoy battles in the Atlantic. Equally, they will prevent the Germans using the island to aid their U-boat campaign.