war to end all wars

1. A war that will be of such huge scale and have such a large impact on the world as to end the need for any other wars in the future. If it ever comes to using nuclear warheads against them, it will truly be a war to end all wars—there won't be any world left to fight another one.

2. A name commonly given to the First World War. My great-grandfather was on the front lines of what was called the war to end all wars, only to see his son march into the fray in a second world war twenty years later.

a war to end all wars

a war, especially the First World War, regarded as making subsequent wars unnecessary.