World War - I Puzzle

A grandfather is telling his grandson stories of world war.

He said :

"At the end of World War 1, I was awarded for my bravery
after saving a group of my men"

"We were fighting in northern France and one of our
enemies threw a grenade at us. I managed to pick it up and
throw it away before it exploded. So right after the war
ended, a General gave me a sword, engraved with the words
'Awarded for Bravery and Valor, A True Hero, World War 1'".

"The grandson thinks about the story for a minute and then
says "Grandpa, that story can't be true!"

How did the grandson know?