Yet more testament to Spartan testicular fortitude.
 

Philip II, father of Alexander the Great, having subjected the other Greek poleis to Macedonian power, threatened the Spartans: "You are advised to submit without further delay, for if I bring my army into your land, I will destroy your farms, slay your people, and raze your city."

The Spartan Response:

"If."

And this was after Leuctra in 371 - when Spartan power was clearly in decline. Philip would have destroyed them (Antipater, Macedon's regent, did in 330) yet the Spartans still refused to back down.

Great story.

It's worth noting, though, that neither Philip nor Alexander bothered them.