Austria-Hungary declared war on Serbia on July 28, 1914, after the assassination of Archduke Franz Ferdinand of Austria.

The Treaty of Versailles was signed on June 28, 1919, between the Allied Powers and Germany, and it officially ended World War I.

Germany invaded Poland on September 1, 1939, which marked the beginning of World War II.

The Molotov-Ribbentrop Pact was signed on August 23, 1939, between Germany and the Soviet Union, and it divided Poland between the two countries.

Great Britain declared war on Germany on September 3, 1939, after Germany invaded Poland.

The Tehran Conference was held in Tehran, Iran, from November 28 to December 1, 1943, and it was attended by the leaders of the Allied Powers: Franklin D. Roosevelt, Winston Churchill, and Joseph Stalin.

The Yalta Conference was held in Yalta, Crimea, from February 4 to 11, 1945, and it was attended by the leaders of the Allied Powers: Franklin D. Roosevelt, Winston Churchill, and Joseph Stalin.

The Potsdam Conference was held in Potsdam, Germany, from July 17 to August 2, 1945, and it was attended by the leaders of the Allied Powers: Harry S. Truman, Winston Churchill, and Joseph Stalin.

The United States dropped an atomic bomb on Hiroshima, Japan, on August 6, 1945, and another atomic bomb on Nagasaki, Japan, on August 9, 1945.

The Treaty of San Francisco was signed on September 8, 1951, between the Allied Powers and Japan, and it officially ended World War II.

The United States was the first country to join the United Nations, on October 24, 1945.

The United Nations was created to promote international cooperation and prevent future wars.

Germany was the first country to leave the League of Nations, in 1933.

Italy was the first country to join the Axis Powers, in 1936.

Great Britain was the first country to join the Allied Powers, in 1939.