In June 1940 Nazi Germany overran France and forced the British army to evacuate at Dunkirk. Severely lacking in military equipment, Britain and its Empire now stood alone against Adolf Hitler's forces. But new Prime Minister Winston Churchill refused to agree to peace terms, forcing Hitler to plan an invasion - codenamed Operation Sea Lion. To stand any chance of crossing the English Channel, Germany would have to crush the Royal Air Force and gain control of the skies during that summer. The Battle of Britain, the first major battle to be decided entirely by air power, had begun.