(CNN) -

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 countries' economies. Although the European economy had stabilized by the 1920s, the Great Depression in the United States 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 formed a coalition called the Axis Powers. Bulgaria, Hungary, Romania, and two German-created states--Croatia and Slovakia--eventually joined.

Major Players: Germany - Adolf Hitler, Der Furher Japan - Admiral Hideki Tojo, Prime Minister Italy - Benito Mussolini, Prime Minister

Allied Powers: The United States, Great Britain, China and the Soviet Union made 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: United States - Franklin D. Roosevelt, President Great Britain - Winston Churchill, Prime Minister China - Chiang Kai-Shek, General Soviet Union - Joseph Stalin, General

U.S. 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.

It is believed that approximately six million Jewish people died in Nazi concentration camps during the war. Also killed were hundreds of thousands of Roma people and people with mental or physical disabilities.

The Lend-Lease Act was created to allow the United States 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 United States created the Marshall Plan to help rebuild war torn Europe. Eventually, 18 nations received $13 billion in food, machinery and other goods.

In March of 1974, Hiroo Onoda, a Japanese soldier still fighting the war, was found by a search party on the island of Lubang in the Philippines. After he is convinced the war is over by his former commanding officer he is then flown to Manila and formally surrenders to President Ferdinand Marcos. Mr. Onoda died January 16, 2014, at the age of 91.

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.