British Expeditionary Force (World War II) in the context of "Invasion of Poland"

The military history of the United Kingdom in World War II covers the Second World War against the Axis powers, starting on 3 September 1939 with the declaration of war by the United Kingdom and France, followed by the UK's Dominions, Crown colonies and protectorates on Nazi Germany in response to the invasion of Poland by Germany. There was little, however, the Anglo-French alliance could do or did do to help Poland. The Phoney War culminated in April 1940 with the German invasion of Denmark and Norway. Winston Churchill became prime minister and head of a coalition government in May 1940. The defeat of other European countries followed – Belgium, the Netherlands, Luxembourg and France – alongside the British Expeditionary Force which led to the Dunkirk evacuation in June 1940.

Britain and its Empire continued the war against Germany. Churchill engaged industry, scientists and engineers to advise and support the government and the military in the prosecution of the war effort. Germany's planned invasion of the UK was averted by the Royal Air Force denying the Luftwaffe air superiority in the Battle of Britain, and by its marked inferiority in naval power. Subsequently, urban areas in Britain suffered heavy bombing during the Blitz in late 1940 and early 1941. The Royal Navy sought to blockade Germany and protect merchant ships in the Battle of the Atlantic. The Army counter-attacked in the Mediterranean and Middle East, including the North-African and East-African campaigns, and in the Balkans.

The Dunkirk evacuation, codenamed Operation Dynamo and also known as the Miracle of Dunkirk, or just Dunkirk, was the evacuation of more than 338,000 Allied soldiers during the Second World War from the beaches and harbour of Dunkirk, in the north of France, between 26 May and 4 June 1940. The operation began after large numbers of Belgian, British, and French troops were cut off and surrounded by German troops during the six-week Battle of France.

After Germany invaded Poland in September 1939, France and the British Empire declared war on Germany and imposed an economic blockade. The British Expeditionary Force (BEF) was sent to help defend France. After the Phoney War of October 1939 to April 1940, Germany invaded Belgium, the Netherlands, and France on 10 May 1940. Three panzer corps attacked through the Ardennes and drove northwest to the English Channel. By 21 May, German forces had trapped the BEF, the remains of the Belgian forces, and three French field armies along the northern coast of France. BEF commander General Viscount Gort immediately saw evacuation across the Channel as the best course of action, and began planning a withdrawal to Dunkirk, the closest good port.

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.

The Battle of Dunkirk (french: Bataille de Dunkerque) was fought around the French port of Dunkirk (Dunkerque) during the Second World War, between the Allies and Nazi Germany. As the Allies were losing the Battle of France on the Western Front, the Battle of Dunkirk was the defence and evacuation of British and other Allied forces to Britain from 26 May to 4 June 1940.

After the Phoney War, the Battle of France began in earnest on 10 May 1940. To the east, the German Army Group B invaded the Netherlands and advanced westward. In response, the Supreme Allied Commander, French General Maurice Gamelin, initiated "Plan D" and British and French troops entered Belgium to engage the Germans in the Netherlands. French planning for war relied on the Maginot Line fortifications along the German–French border protecting the region of Lorraine but the line did not cover the Belgian border. German forces had already crossed most of the Netherlands before the French forces had arrived. Gamelin instead committed the forces under his command – three mechanized forces, the French First and Seventh Armies and the British Expeditionary Force (BEF) – to the River Dyle. On 14 May, German Army Group A burst through the Ardennes and advanced rapidly westward toward Sedan, turning northward to the English Channel, using Generalfeldmarschall Erich von Manstein's plan Sichelschnitt (under the German strategy Fall Gelb), effectively flanking the Allied forces.

The Phoney War (French: Drôle de guerre; German: Sitzkrieg; Polish: Dziwna wojna) was an eight-month period at the outset of World War II, during which there were virtually no Allied military land operations on the Western Front from roughly September 1939 to May 1940.

World War II began on 1 September 1939 with the invasion of Poland by Germany and the Soviet Union. Formal declarations of war by the United Kingdom and France followed on 3 September, marking the start of the so-called "Phoney War" period with little actual warfare occurring.