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 (Operation Sea Lion) 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. 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 and anti-tank cubes are commonly found.