Anonymous
After 1945, historians found the term "First World War" appropriate because they saw 1914-1918 as the first of a particular type of international conflict - the world's first industrialized "total" war - which had been followed by a second industrialized world war of this kind - 1939-1945.
Anonymous
Probably circa 1546
Sir Caustic
1913. No-one's been able to find out how that happened, but it did. Most historians think it's got something to do with space-time swirly things.
Fried Potatoes
When they had the second one