Germany invades Poland Nazi Germany invades Poland, prompting Britain and France to declare war on Germany. The United States declares neutrality on September 5. The Soviet Union will invade Poland from the east on September 17, and on September 29 Germany and the Soviet Union will divide up Poland.

Germany invades Western Europe Germany invades France, Belgium, Luxembourg and the Netherlands. On the same day, Winston Churchill becomes British prime minister. The Germans will enter Paris on June 14 and will occupy most of France for the next four years.

Battle of Britain Germany begins a long series of air assaults on Great Britain. The initial targets are transportation centers and aircraft factories, but on August 23 the Germans bomb Central London. The British retaliate by bombing Berlin on August 25. During a four-month campaign known as the Battle of Britain, the Germans will fail to disable Britain’s air defenses. The British will continue to launch intermittent air attacks on German cities — most notably Hamburg, Berlin and Dresden — for the rest of the war.

Germany invades the Soviet Union In violation of the August 1939 Molotov-Ribbentrop Pact—a treaty of non-aggression between Germany and the Soviet Union — Hitler launches a massive assault, known as Operation Barbarossa, on the Soviet Union. Germany is now fighting a war on two fronts.

Japanese attack Pearl Harbor Japan launches a surprise air assault on the U.S. air base at Pearl Harbor, Hawaii, sinking four battleships, destroying 188 aircraft and killing 2,402 Navy personnel. The following day the United States and Britain declare war on Japan. On December 11, Germany declares war on the United States.

German U-Boat offensive The Germany Navy launches a series of nighttime attacks along the Atlantic coast of the United States, sinking 348 ships over a four-month period. During this time the U.S. Navy manages to sink only two German submarines (U-Boats).

Bataan Death March Japanese troops on the island of Bataan, in the Philippines, force 76,000 Allied prisoners to walk 60 miles in scorching heat without food or water to a POW camp. Of the 12,000 Americans who begin the march, more than 5,000 will die during the 10-day ordeal.

Battle of Midway Following on the heels of the first Allied victory against the Japanese — in the Battle of the Coral Sea in May 1942 — the Battle of Midway is a turning point in the war in the Pacific. The United States achieves a dramatic win, destroying four Japanese aircraft carriers.

Battle of Guadalcanal The U.S. Marines make their first amphibious landing in the Pacific, invading Tulagi and Gaudalcanal in the Solomon Islands. More than 7,000 Americans will die during a brutal but ultimately successful six-month campaign.

Allied Invasion of North Africa Operation Torch begins, as British and American troops land in Morocco and Algeria. Over the next year, the Allies will wrest control of North Africa from the Axis powers, allowing the Allies to use North Africa as a staging ground for the invasion of Italy in July 1943.

D-Day: Allied Invasion of France The Allies launch the largest amphibious invasion in history, landing 175,000 troops on the beaches of Normandy in northern France. Known as Operation Overlord, the invasion is the beginning of an 11-month campaign to free Western Europe from Nazi control.

Battle of Leyte Gulf The Allies deal the Imperial Japanese Navy a debilitating blow during a four-day battle — the largest Naval battle of the war — in the Leyte Gulf near the Philippine islands of Leyte, Samar and Luzon. During this battle Japanese Kamikazi pilots launch the first air suicide attacks against U.S. ships.

Battle of the Bulge Hitler launches a massive counter-offensive in the Argonne Forest in eastern France, creating a “bulge” in the Allied lines. During 12 days of intense fighting, 19,276 Americans will die — more than in any other European battle of WWII.

Invasion of Iwo Jima U.S. marines land on Iwo Jima, a volcanic island where the Japanese have built an immense network of heavily fortified underground bunkers, with the goal of capturing two strategically important airfields. It will take 110,000 marines five weeks to neutralize the island’s 18,000 Japanese defenders, at a cost of 6,821 American lives.

Invasion of Okinawa The Allies launch the largest amphibious operation of the Pacific War, invading the Japanese island of Okinawa. The ensuing 82-day battle will claim 12,513 American lives, making it the third deadliest battle in U.S. military history (after the 1918 Battle of Meuse-Argonne and the 1944 Battle of the Bulge).

Franklin Roosevelt dies U.S. President Franklin Delano Roosevelt dies in Warm Springs, Georgia, less than three months into his fourth term as president. Vice President Harry Truman becomes commander-in-chief and learns about the secret program to develop an atomic bomb. The same day that Roosevelt dies, the U.S. 89^th Infantry liberates the Nazi concentration camp at Buchenwald—the first of dozens of concentration camps that U.S. troops will discover during their march through Germany.

Demise of Hitler and Mussolini Nine days after Soviet troops arrived in Berlin, Hitler commits suicide in his underground bunker, thus avoiding the fate of Italian dictator Benito Mussolini, who was executed by Italian partisans on April 27.

Germany surrenders Germany’s top military leaders surrender unconditionally to the Allies. The following day the world celebrates Victory in Europe Day.

Atomic bombing of Hiroshima Hoping to preempt an invasion of Japan that might have cost hundreds of thousands of American lives, the United States drops an atomic bomb on Hiroshima. About 80,000 Japanese civilians are killed instantly. Japan refuses to surrender. Three days later the United States launches an atomic assault on Nagasaki, killing almost 74,000 people.