World War II, or abbreviated as WWII or WW2, was the most destructive engagment on Earth, that was fought between 1939 and 1945.
World War II was the second global military conflict in human history. It was fought between the Axis powers and the Allies.
The war started in 1939 (this date is somewhat disputed - the Japanese had been fighting the Chinese sporadically since 1931, and an all-out war began in 1937) when Adolf Hitler, Führer of Nazi Germany, invaded Poland. Britain and France had allying pacts with Poland, and so both nations declared war on Nazi Germany. However, in 1940, during a disastrous retreat of the French 9th army from the Ardennes forest region, Army Group A broke into France, and the resulting blitzkrieg offensive quickly overtook the rest of the defenders. The Axis immediately headed Northward to surround Allied forces. The BEF (British expeditionary force) as well as the French army, retreated to a small pocket around Calais and Dunkirk. The slowly shrinking pocket was evacuated by a Royal Navy force of 'Little ships', and around 338,000 men were evacuated.
In 1941, three German army Groups (South, Centre and North) numbering 3,900,000 men and 3,600 tanks, rolled over Soviet controlled territory and began an offensive toward Moscow. The Red Army, outnumbering the Germans massively, was forced back into Russia, and by the winter of 1941, the Germans had reached the suburbs of Moscow. Meanwhile, in December 1941, a Japanese carrier task force began a huge aerial assault on the US naval base of Pearl Harbor. They sunk or seriously damaged 21 ships, including 8 battleships, and brought the USA onto the Allied side of the war. The Japanese at the same time began attacking British, French, and Dutch colonies, American territories, and Australia.
1942 saw the Americans land in northern Africa, and with the British 8th Army, they pushed the German Afrika Corps out of North Africa and into Italy. Italy was invaded in 1943, with two British armies, and three American. Fighting in Italy would not subside until 1945.
In 1943, US and British paratroopers landed in Italy as operation Husky an Allied effort to liberate Italy, as the Italian civilians didn't like Mussolini he just rose to power. The operation was a success but at a high price, but was worth the effort. By the end of 1943 the Allies had reached Rome and had liberated Italy but minor resistance from the Italians fell back with the German side.
In 1944, 153,000 American, British Commonwealth, Free French, and Polish troops landed on 5 separate beaches (west to east: Utah, Omaha, Gold, Juno, Sword) and dropped into three separate landing zones in Normandy. German resistance in some areas was strong, with American losses on Omaha beach reaching 3,000 dead, wounded or missing. However the Germans were forced back, and hundreds of thousands of German equipment and troops, were surrounded near Falaise. The surrounded troops, were picked off by superior allied aircraft, where they were bombarded with bullets bombs and rockets. By early 1945, the Allies had invaded Germany from the east and west.
In the Pacific, where American forces were 'island hopping' their way to Japan, they had successfully captured the Philippines, as well as New Guinea, Rabaul, the Solomon Islands, the Marianas, and were moving to attack Iwo Jima. The British had also began a large offensive in Burma, and were pushing the Japanese back into China.
In April 1945, after beating back the German forces through occupied Soviet territory and Eastern Europe, the Red Army surrounded the German capital of Berlin. Hitler committed suicide, and he appointed Karl Donitz to be the new Führer. They refused to surrender, and so the Soviets bombarded the city and sent wave after wave of troops into the city to capture it. They didn't capture it until May 8, when General Alfred Jodl signed the unconditional surrender for all German forces to the Allies.
In the Pacific, the Americans had managed to take control of Iwo Jima, at a heavy cost. It was one of the few instances where American casualties exceeded the Japanese, 22,000 men were killed or wounded. The Americans would then attack Okinawa, the last island needed for an invasion of Japan. The Japanese Navy, now reduced to a tiny force, launched kamikaze suicide aircraft at the US invasion fleet.
US generals knew that an invasion of Japan would be a costly enterprise. The defenders would attack with such fanaticism that US casualties were expected to exceed 1,000,000 men and Japanese casualties were expected to be in the tens of millions, with the nation's infrastructure almost totally destroyed. The planned invasion would land millions of Allied soldiers onto the Japanese islands, which would have made it the largest ever attempted naval invasion, dwarfing Operation Overlord, the invasion of Normandy, but it never happened. Two B-29 Superfortresses took off from their bases in the Marianas and dropped an atomic bomb on the city of Hiroshima on August 6 and three days later on Nagasaki. Several days later, the Japanese formally surrendered aboard the battleship, USS Missouri.
The war formally ended on September 2, 1945, six years and one day after war was declared. By the end, 78,000,000 people had died, most of them civilians.