The Sitzkrieg was a phase in early World War II from September 1939 until May 1940 marked by few military operations in Continental Europe, in the months following the German invasion of Poland and preceding the Battle of France. The great powers of Europe had declared war on one another, yet neither side had committed to launching a significant attack, and there was relatively little fighting on the ground. While most of the German army was fighting against Poland, a much smaller German force manned the Siegfried Line, their fortified defensive line along the French border. At the Maginot Line on the other side of the border, British and French troops stood facing them, but there were only some local, minor skirmishes. The British Royal Air Force dropped propaganda leaflets on Germany and the first Canadian troops stepped ashore in Britain, while western Europe was in a strange calm for seven months.

The Battle of the Atlantic was the longest continuous military campaign of World War II, running from 1939 through the defeat of Nazi Germany in 1945, and was at its height from mid-1940 through to about the end of 1943.
The Battle of the Atlantic pitted U-boats and other warships of the German Navy (Kriegsmarine) against Allied convoys. The convoys, coming mainly from North America and the South Atlantic and going to the United Kingdom and the Soviet Union, were protected for the most part by the British and Canadian navies and air forces. These forces were later aided by ships and aircraft of the United States. The German warships were joined by submarines of the Italian Royal Navy (Regia Marina) after Italy entered the war on 10 June 1940.The name "Battle of the Atlantic", first coined by Winston Churchill in 1941, is a partial misnomer for a campaign that began on the first day of the European war and lasted for six years, involved thousands of ships and stretched over hundreds of miles of the vast ocean and seas in a succession of more than 100 convoy battles and perhaps 1,000 single-ship encounters. Tactical advantage switched back and forth over the six years as new weapons, tactics and counter-measures were developed by both sides. The British and their allies gradually gained the upper hand, driving the German surface raiders from the ocean by the middle of 1941 and decisively defeating the U-boats in a series of convoy battles between March and May 1943. New German submarines arrived in 1945, but they were too late to affect the course of the war.

Operation Weserübung was the German codename for Nazi Germany's assault on Denmark and Norway during World War II and the opening operation of the Norwegian Campaign. The name comes from the German for Operation Weser-Exercise, the Weser being a German river. In the early morning of April 9, 1940 — Wesertag — Germany invaded Denmark and Norway, ostensibly as a preventive maneuver against a supposed planned Franco-British occupation of Norway. After the invasions, envoys of the Germans informed the governments of Denmark and Norway that the Wehrmacht had come to protect the countries' neutrality against Franco-British aggression. Significant differences in geography, location and climate between the two countries made the actual military operations very dissimilar