Archduke Franz Ferdinand, heir to Austria-Hungary's throne, and his wife, Sophie, are assassinated
Austria-Hungary declares war on Serbia.
Germany declares war on Russia.
Germany declares war on France
The United Kingdom declares war on Germany, after Germany invades Belgium.
Austria-Hungary declares war on Russia and Serbia declares war on Germany.
U.S. President Woodrow Wilson announces the U.S. will remain neutral
The Battle of Tannenberg begins.
The First Battle of the Marne begins. Trench warfare begins as soldiers on both sides dig in.
Battle of Ypres begins.
The United Kingdom announces that the North Sea is a military area, effectively creating a blockade of goods into Germany
The unofficial Christmas truce is declared.
Germany declares a "war zone" around Great Britain, essentially effecting a submarine blockade where even neutral merchant vessels were to be potential targets.
The Dardanelles Campaign begins
The Second Battle of Ypres begins. It is during this battle that the Germans first use poison gas.
The Battle of Gallipoli begins.
The British ocean liner RMS Lusitania is sunk by German U-boat, U-20
Tsar Nicholas II takes personal control over Russia's armies.
Germany sends the secret Zimmerman Telegram to Mexico in an effort to entice Mexico to join the war. The British intercept and decipher the coded message.
The Battle of Verdun begins. The Battle of Verdun was the longest battle of World War I and was one of the bloodiest.
The United States declares war on Germany
U.S. President Woodrow Wilson issues his Fourteen Points to peace.