While the Royal Air Force defended the skies, and convoys of ships braved German attacks in the Atlantic to bring supplies to Britain from North America, Hitler changed his plans, and decided to break his pact with Stalin and invade Russia. For a time, his armies were unstoppable.
On the other side of the world, Japan joined the side of Germany, and invaded much of South East Asia, capturing the British in Singapore and threatening India and Australia.
British and Commonwealth troops (“the Allies”) fought the Germans and Italians in the deserts of North Africa, where early successes turned to near defeat.
Three events then turned the tide of war. The Russians defeated the Germans at Stalingrad, the Allies won the battle of El Alamein in the desert, and the Japanese made a surprise attack on American ships at Pearl Harbour in the Pacific, which led to the United States joining the Allies.