British anti-invasion preparations of World War II

British anti-invasion preparations of the Second World War entailed a large-scale division of military and civilian mobilisation in response to the threat of invasion by German armed forces in 1940 and 1941. The British army needed to recover from the defeat of the British Expeditionary Force in France, and 1.5 million men were enrolled as part-time soldiers in the Home Guard. The rapid construction of field fortifications transformed much of the United Kingdom, especially southern England, into a prepared battlefield. The German invasion plan, Operation Sea Lion, was never taken beyond the preliminary assembly of forces. Today, little remains of Britain's anti-invasion preparations. Only reinforced concrete structures such as pillboxes are common.

On 1 September 1939, Germany invaded Poland; two days later, Britain and France declared war on Germany, launching the Second World War. Within three weeks, the Red Army of the Soviet Union invaded the eastern regions of Poland in fulfilment of the secret Molotov-Ribbentrop Pact with Germany. A British Expeditionary Force (BEF) was sent to the Franco-Belgian border, but Britain and France did not take any direct action in support of the Poles. By 1 October, Poland had been completely overrun.

There was little fighting over the months that followed. In a period known as the Phoney War, soldiers on both sides trained for war and the French and British constructed and manned defences on the eastern borders of France.

...
Wikipedia