On April 9, 1940, Nazi Germany invaded the neutral nations of Norway and Denmark.
Synopsis from WWII Database.com:
German troops crossed into Denmark at 0500 hours, with landings near Copenhagen unopposed; the Danish government surrendered within the same day, and Germany completed the conquest Denmark with only 20 casualties.
To the north in Norway, German troops attacked four locations. At Narvik, German destroyers sank Norwegian coastal cruisers Eidsvold and Norge, killing 276. At Trondheim, German warships pretended to be British ships and sailed by the coastal batteries without being hassled, thus the city was captured with relative ease. At Bergen, the coastal batteries at Fort Kvarven damaged German cruiser Königsberg and minelayer Bremse.
Off Bergen, German Ju 88 and He 111 aircraft attacked British battleship HMS Rodney and destroyer HMS Gurkha at 1400 hours; Rodney was hit by a dud 500-kg bomb, and Gurkha sank at 1600 hours, killing 15; only four German Ju 88 aircraft were lost in this attack.
Finally, at Oslo, the batteries at Oscarborg sank German cruiser Blücher in the Oslofjord, killing 830. Out at sea, British battlecruiser HMS Renown intercepted German battleships Scharnhorst and Gneisenau after they had successfully escorted Marine Gruppe 1 to Narvik; Renown fired first, hitting Gneisenau three times, but received two hits before the German ships disengaged from the battle.
Given the dire situation, the Norwegian royal family, the government, and the country’s gold reserves (with over 48 tons of gold) departed from Oslo at 0830 hours.
British submarine HMS Truant torpedoed and heavily damaged German cruiser Karlsruhe in the Skagerrak at 1957 hours. After the crew was rescued, German torpedo boat Grief scuttled Karlsruhe.
History.com adds some details of the invasion:
On this day in 1940, German warships enter major Norwegian ports, from Narvik to Oslo, deploying thousands of German troops and occupying Norway. At the same time, German forces occupy Copenhagen, among other Danish cities.
German forces were able to slip through the mines Britain had laid around Norwegian ports because local garrisons were ordered to allow the Germans to land unopposed. The order came from a Norwegian commander loyal to Norway’s pro-fascist former foreign minister Vidkun Quisling.
Hours after the invasion, the German minister in Oslo demanded Norway’s surrender. The Norwegian government refused, and the Germans responded with a parachute invasion and the establishment of a puppet regime led by Quisling (whose name would become a synonym for “traitor”). Norwegian forces refused to accept German rule in the guise of a Quisling government and continued to fight alongside British troops. But an accelerating German offensive in France led Britain to transfer thousand of soldiers from Norway to France, resulting ultimately in a German victory.
In Denmark, King Christian X, convinced his army could not fight off a German invasion, surrendered almost immediately. Hitler now added a second and third conquered nation to his quarry, which began with Poland.