On July 5, 1914, Kaiser Wilhelm II of Germany pledged his country's complete support for whatever response Austria-Hungary would choose to make against Serbia after the June 28th assassination of Archduke Franz Ferdinand of Austria by a Serbian nationalist during an official visit to Sarajevo, Bosnia. This fatal error went down in history as Germany's carte blanche or "blank check," assurance to Austria that led directly to WW I.
In September 1939, World War II began when Great Britain and France came to the assistance of Poland after the German Army invaded, fulfilling a "guarantee" made in March of that year. What was a regional war, and one that might have been resolved through diplomacy, became global.
One would think that after such commitments were assessed by historians as the immediate causes of two world wars, no one would ever consider going down that road again. But that would be reckoning without Republican Senator Lindsey Graham who has been calling for a "defense treaty" with Israel since last April.