The assassination of Archduke Franz Ferdinand, heir to the Austro-Hungarian throne, on June 28, 1914, triggered a chain of events that led to the outbreak of World War I.

Austria-Hungary declared war on Serbia on July 28, 1914, after Serbia refused to accept an ultimatum following the assassination of Archduke Franz Ferdinand.

Russia suffered the most casualties in World War I, with an estimated 1.7 million soldiers killed and 5.5 million wounded.

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

Germany invaded Poland on September 1, 1939, marking the start of World War II.

The Soviet Union suffered the most casualties in World War II, with an estimated 27 million deaths.

Little Boy was the name of the atomic bomb dropped on Hiroshima, Japan, on August 6, 1945.

Fat Man was the name of the atomic bomb dropped on Nagasaki, Japan, on August 9, 1945.

Italy surrendered to the Allies on September 8, 1943, becoming the first Axis power to surrender in World War II.

Japan surrendered to the Allies on September 2, 1945, after the atomic bombings of Hiroshima and Nagasaki.

The Yalta Conference was held in Yalta, Crimea, from February 4 to 11, 1945, and was attended by the leaders of the United States, the United Kingdom, and the Soviet Union.

The Potsdam Conference was held in Potsdam, Germany, from July 17 to August 2, 1945, and was attended by the leaders of the United States, the United Kingdom, and the Soviet Union.

The United Nations was established on October 24, 1945, with the signing of the United Nations Charter in San Francisco, California.

The United States was the first country to develop an atomic bomb, with the Manhattan Project successfully detonating the first atomic bomb in New Mexico on July 16, 1945.

The Soviet Union was the second country to develop an atomic bomb, successfully detonating its first atomic bomb in Kazakhstan on August 29, 1949.