CNN — About 50,000 people from Japan and abroad came to Hiroshima to commemorate the 68th anniversary of the atomic bombings. A memorial service was held in the Hiroshima Peace Memorial Park, marking the day the US military dropped an atomic bomb on the city on August 6th in 1945. The names of victims are stored in a stone vault. They now total about 280,000. This year 5,800 names were added for the elderly survivors who died in the past 12 months.
Here's what you need to know about World War II, which lasted from 1939 to 1945.
Causes of World War II: The Peace of Paris - The treaties worked out in Paris at the end of World War I satisfied few. Germany, Austria, and the other countries on the losing side of the war were especially unhappy with the Paris Agreement, which required them to give up arms and make reparations. Germany agreed to sign the Treaty of Versailles only after the victorious countries threatened to invade if Germany did not sign it.
Economic Issues - World War I was devastating to country economies. Although the European economy had stabilized by the 1920s, the Great Depression in the U.S. led to economic downfall in Europe. Communism and fascism gained strength in the wake of economic problems.
Nationalism - An extreme form of patriotism that grew in Europe became even stronger after World War I, especially for countries that were defeated.
Dictatorships - Political unrest and unfavorable economic conditions lead to the rise of dictatorships in countries such as Germany, Italy, Japan and the Soviet Union.
Failure of Appeasement - Czechoslovakia had become an independent nation after World War I, but by 1938, was surrounded by German territory. Hitler wanted to annex the Sudetenland, an area in western Czechoslovakia where many Germans lived. British Prime Minister Neville Chamberlain wanted to appease Hitler and agreed to his demands for the Sudetenland after Hitler promised he would not demand more territory. Hitler seized the rest of Czechoslovakia in March of 1939.
Axis Powers: Germany, Japan, and Italy form a coalition called the Axis Powers. Bulgaria, Hungary, Romania, and two German-created states--Croatia and Slovakia--eventually join.
Major players: Germany - Adolf Hitler, Der Furher Japan - Admiral Hideki Tojo, Prime Minister Italy - Benito Mussolini, Prime Minster
Allied Powers: The United States, Great Britain, China and the Soviet Union make up the Allies, the group fighting the Axis. Between 1939 and 1944 at least 50 nations would eventually fight together. Thirteen more nations would join by 1945 including: Australia, Belgium, Brazil, British Commonwealth of Nations, Canada, India, New Zealand, South Africa, Czechoslovakia, Denmark, France, Greece, Netherlands, Norway, Poland, Philippines, and Yugoslavia.
Major players: U.S. - Franklin D. Roosevelt, President Great Britain - Winston Churchill, Prime Minister China - Chiang Kai-Shek, General Soviet Union - Joseph Stalin, General
US Troop Statistics: 16,112,566 - Number of U.S. troops that served in the conflict. 670,846 - Number of U.S. wounded.
U.S. Deaths: Battle: 291,557 Non-Battle: 113,842 Total In-Theatre: 405,399
Other Military Casualties by Country 1939-1945 (selected): Australia: 23,365 dead; 39,803 wounded Austria: 380,000 dead; 350,117 wounded Belgium: 7,760 dead; 14,500 wounded Bulgaria: 10,000 dead; 21,878 wounded Canada: 37,476 dead; 53,174 wounded China: 2,200,000 dead; 1,762,000 wounded France: 210,671 dead; 390,000 wounded Germany: 3,500,000 dead; 7,250,000 wounded Great Britain: 329,208 dead; 348,403 wounded Hungary: 140,000 dead; 89,313 wounded Italy: 77,494 dead; 120,000 wounded Japan: 1,219,000 dead; 295,247 wounded Poland: 320,000 dead; 530,000 wounded Romania: 300,000 dead; wounded unknown Soviet Union: 7,500,000 dead; 5,000,000 wounded United States: 405,399 dead; 670,846 wounded
Other Facts: About 70 million people fought in the armed forces of the Allied and Axis nations.
Finland never officially joined either the Allies or the Axis and was at war with the Soviet Union at the outbreak of World War II. Needing help in 1940, the Finnish joined forces with Nazi Germany to repel the Soviets. When peace between Finland the Soviet Union was declared in 1944, Finland joined with the Soviets to oust the Germans.
Switzerland, Spain, Portugal and Sweden declared neutrality during the war.
The Soviet Union lost the most soldiers, in excess of seven million.
The number of civilian casualties in World War II may never be known. Many deaths were caused by bombing raids, massacres, starvation and other war-related causes.
The Lend-Lease Act was created to allow the U.S. to lend or lease weapons, equipment, or raw materials to any nation fighting the Axis. Eventually, 38 nations received about $50 billion in aid. Most went to Great Britain and the Soviet Union.
In 1948, the U.S. created the Marshall Plan to help rebuild war torn Europe. Eventually, 18 nations received $13 billion in food, machinery and other goods.
Timeline: September 1, 1939 - Germany invades Poland. Denmark, Luxembourg, the Netherlands, Norway, Belgium, and France soon fall into German control, until only the United Kingdom is left to face Germany.
June 10, 1940 - Italy joins the war on the side of Germany by declaring war against Britain (UK) and France. Fighting spreads to Greece and Northern Africa.
June 14, 1940 - German troops march into Paris.
July 1940-September 1940 - Germany and Great Britain fight an air war, the Battle of Britain, along the English coastline.
September 7, 1940-May 1941 - German bombing campaign of nightly air raids over London, known as the Blitz.
January 22, 1941 - British and Commonwealth troops take over the port city of Tobruk, Libya.
June 22, 1941 - Germany invades the Soviet Union.
September 1941 -- Japanese troops invade Indochina.
December 7, 1941 - Japan attacks Pearl Harbor, destroying more than half of the fleet of aircraft, and damaging all eight battleships. Japan also attacks Clark and Iba airfields in the Philippines destroying over half of the U.S. Army's aircraft there.
December 8, 1941 - President Franklin D. Roosevelt delivers the "a date which will live in infamy" speech to Congress, and the U.S declares war on Japan. Japan invades Hong Kong, Guam, the Wake Islands, Singapore, and British Malaya.
December 11, 1941 - Germany and Italy declare war on the U.S.
By Christmas 1941, Japan had taken Thailand, Guam, Hong Kong, and Wake Island.
1942 - The Allies stop the Axis Powers' advance in Northern Africa and the Soviet Union.
February 1942 - Japan invades the Malay Peninsula. Singapore surrenders within a week.
June 4-6, 1942 - Japan's plans to invade the Hawaiian Islands, starting at Midway Island, but the U.S. cracks the code of the mission. Japan attacks Midway and loses four aircraft carriers and over 200 planes and pilots in the first clear victory for the U.S.
August 19, 1942 - Battle for Stalingrad begins as Germany pushes further into Russia.
August 1942-February 1943 - U.S. Marines fight for and hold the Pacific island of Guadalcanal.
October 23, 1942 - British troops push Axis troops into retreating to Tunisia in the Second Battle of El Alamein.
February 1, 1943 - The German troops in Stalingrad surrender, defeated in large part by the Soviet winter. The defeat marks the halt of Germany's eastbound advance.
July 10, 1943 - Allied forces land in Italy.
July 25, 1943 - The King of Italy is restored to full power and Mussolini is deposed and arrested.
November 1943-March 1944 - U.S. Marines invade the Solomon Islands at Bougainville to recapture it from the Japanese.
June 6, 1944 - D-Day, in which Allied forces land on five beaches at Normandy: Utah, Omaha, Gold, Juno and Sword. The landing includes over 5,000 ships, 11,000 airplanes and over 150,000 service men.
August 25, 1944 - American and Free French forces liberate Paris.
February 19-March 26, 1945 - U.S. Marines battle the Japanese for the island of Iwo Jima.
April 12, 1945 - President Roosevelt dies in Warm Springs, Georgia. Vice President Harry Truman takes the oath of office as president.
April 25, 1945 - Soviet troops surround Berlin.
April 28, 1945 - Mussolini is killed attempting to escape to Switzerland.
April 30, 1945 - Hitler and wife Eva Braun commit suicide.
May 7, 1945 - Germany surrenders in a red school house in Reims, Germany, Eisenhower's headquarters. V-E Day is celebrated on May 8 because that was the day the armistice went into effect.
May 8, 1945 - V-E Day, Victory in Europe. The war in Europe is officially over.
July 16, 1945 - First successful test of the atomic bomb in Alamogordo, New Mexico.
July 29, 1945 - President Harry Truman warns Japan that the country will be destroyed if it does not surrender unconditionally. Japan continues fighting.
August 6, 1945 - The first atomic bomb used in warfare, nicknamed Little Boy, is dropped on the Japanese city of Hiroshima, killing up to 140,000 people.
August 9, 1945 - After getting no response from the Japanese government after the Hiroshima bombing, a second atomic bomb, nicknamed Fat Man, is dropped on Nagasaki, killing up to 80,000 people.
August 14, 1945 - Japan unconditionally agrees to accept the terms of the Potsdam Declaration and end the war. V-J Day, Victory over Japan is declared.
September 2, 1945 - Japan signs the formal surrender aboard the USS Missouri in Tokyo Bay.
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