Ultimatum
Ultimatum, in diplomacy, a formal demand made by one nation upon another. It requires the receiving government to comply with the demand of the sender, usually by a specified time, or suffer the consequences, usually war. For example, on August 4, 1914, Great Britain demanded that Germany halt its invasion of Belgium. If Germany failed to stop its aggression against Belgium by midnight, the ultimatum stated, Great Britain would go to war against Germany. The demand was rejected, and Britain entered World War I.