The Battle of Boulogne in 1940 was the defence of the port of Boulogne-sur-Mer by French, British and Belgian troops in the Battle of France during the Second World War. The battle was fought at the same time as the Siege of Calais, just before Operation Dynamo, the evacuation of the British Expeditionary Force (BEF) from Dunkirk. After the Franco-British counter-attack at the Battle of Arras on 21 May, German units were held ready to resist a resumption of the attack on 22 May. General der Panzertruppe (Lieutenant-General) Heinz Guderian, the commander of XIX Corps, protested that he wanted to rush north up the Channel coast to capture Boulogne, Calais and Dunkirk. An attack by part of XIX Corps was not ordered until 12:40 p.m. on 22 May, by which time the Allied troops at Boulogne had been reinforced from England by most of the 20th Guards Brigade.

The Guards had time to dig in around the port before the 2nd Panzer Division, which had been delayed by French troops at Samer, attacked the perimeter held by the Irish Guards at around 5:00 p.m. and were driven off after an hour of fighting. The Welsh Guards front was attacked at 8:00 p.m. and again at dusk, cutting off a party of the Irish at 10:00 p.m. At dawn on 23 May, the German attacks resumed, eventually pushing the defenders back into the town. About eighty light bombers of the Royal Air Force (RAF) flew sorties in support of the defenders of the port.

Royal Navy ships shot their way into and out of the harbour; French and British destroyers bombarded German positions as wounded and non-combatants were embarked and a navy demolition party landed. During a lull in the afternoon of 23 May, the Luftwaffe bombed the harbour, despite being intercepted by RAF fighters. At 6:30 p.m. the Guards Brigade was ordered to re-embark; the British destroyers ran the gauntlet of German tanks and artillery to dock. The French defenders above the lower town could not be contacted and only in the morning of 24 May did General Lanquetot realise that the British had gone.

The French and the remaining British troops held out until 25 May and then surrendered. Guderian wrote that the halt order and the retention of considerable forces to guard against Allied counter-attacks, forfeited an opportunity quickly to capture the Channel Ports and destroy the Allied forces in northern France and Belgium. An advance on Dunkirk began on 23 May but the next day was halted until 27 May; Dunkirk was not captured until 4 June, by when, most of the BEF and many French and Belgian troops had escaped.

World War II or the Second World War, often abbreviated as WWII or WW2, was a global war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing military alliances: the Allies and the Axis powers. In a total war directly involving more than 100 million personnel from more than 30 countries, the major participants threw their entire economic, industrial, and scientific capabilities behind the war effort, blurring the distinction between civilian and military resources. Aircraft played a major role in the conflict, enabling the strategic bombing of population centres and the only two uses of nuclear weapons in war. World War II was by far the deadliest conflict in human history; it resulted in 70 to 85 million fatalities, a majority being civilians. Tens of millions of people died due to genocides (including the Holocaust), starvation, massacres, and disease. In the wake of the Axis defeat, Germany and Japan were occupied, and war crimes tribunals were conducted against German and Japanese leaders.

The exact causes of World War II are debated, but contributing factors included the Second Italo-Ethiopian War, the Spanish Civil War, the Second Sino-Japanese War, the Soviet–Japanese border conflicts and rising European tensions since World War I. World War II is generally considered to have begun on 1 September 1939, when Nazi Germany, under Adolf Hitler, invaded Poland. The United Kingdom and France subsequently declared war on Germany on 3 September. Under the Molotov–Ribbentrop Pact of August 1939, Germany and the Soviet Union had partitioned Poland and marked out their "spheres of influence" across Finland, Estonia, Latvia, Lithuania and Romania. From late 1939 to early 1941, in a series of campaigns and treaties, Germany conquered or controlled much of continental Europe, and formed the Axis alliance with Italy and Japan (along with other countries later on). Following the onset of campaigns in North Africa and East Africa, and the fall of France in mid-1940, the war continued primarily between the European Axis powers and the British Empire, with war in the Balkans, the aerial Battle of Britain, the Blitz of the UK, and the Battle of the Atlantic. On 22 June 1941, Germany led the European Axis powers in an invasion of the Soviet Union, opening the Eastern Front, the largest land theatre of war in history.

Japan, which aimed to dominate Asia and the Pacific, was at war with the Republic of China by 1937. In December 1941, Japan attacked American and British territories with near-simultaneous offensives against Southeast Asia and the Central Pacific, including an attack on the US fleet at Pearl Harbor which resulted in the United States declaring war against Japan. Therefore the European Axis powers declared war on the United States in solidarity. Japan soon captured much of the western Pacific, but its advances were halted in 1942 after losing the critical Battle of Midway; later, Germany and Italy were defeated in North Africa and at Stalingrad in the Soviet Union. Key setbacks in 1943—including a series of German defeats on the Eastern Front, the Allied invasions of Sicily and the Italian mainland, and Allied offensives in the Pacific—cost the Axis powers their initiative and forced it into strategic retreat on all fronts. In 1944, the Western Allies invaded German-occupied France, while the Soviet Union regained its territorial losses and turned towards Germany and its allies. During 1944 and 1945, Japan suffered reversals in mainland Asia, while the Allies crippled the Japanese Navy and captured key western Pacific islands.

The war in Europe concluded with the liberation of German-occupied territories, and the invasion of Germany by the Western Allies and the Soviet Union, culminating in the fall of Berlin to Soviet troops, Hitler's suicide and the German unconditional surrender on 8 May 1945. Following the Potsdam Declaration by the Allies on 26 July 1945 and the refusal of Japan to surrender on its terms, the United States dropped the first atomic bombs on the Japanese cities of Hiroshima, on 6 August, and Nagasaki, on 9 August. Faced with an imminent invasion of the Japanese archipelago, the possibility of additional atomic bombings, and the Soviet’s declared entry into the war against Japan on the eve of invading Manchuria, Japan announced on 15 August its intention to surrender, then signed the surrender document on 2 September 1945, cementing total victory in Asia for the Allies.

World War II changed the political alignment and social structure of the globe. The United Nations (UN) was established to foster international co-operation and prevent future conflicts, with the victorious great powers—China, France, the Soviet Union, the United Kingdom, and the United States—becoming the permanent members of its Security Council. The Soviet Union and the United States emerged as rival superpowers, setting the stage for the nearly half-century-long Cold War. In the wake of European devastation, the influence of its great powers waned, triggering the decolonisation of Africa and Asia. Most countries whose industries had been damaged moved towards economic recovery and expansion. Political and economic integration, especially in Europe, began as an effort to forestall future hostilities, end pre-war enmities and forge a sense of common identity.