The King of France marches twenty thousand men up a hill and, in true nonsense-rhyme fashion, never manages to go up again. The joke is the anticlimax, much like the Grand Old Duke of York.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
The King of France went up the hill,
With twenty thousand men;
The King of France came down the hill,
And ne'er went up again.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition