Via the Wikipedia entry on Laconic phrase. This must be the ultimate example.
Direct quote from Wikipedia:
From the time of the invasion of Philip II of Macedon. With key Greek city-states in submission, he turned his attention to Sparta and sent a message: “If I win this war, you will be slaves forever.” In another version, Philip proclaims: “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 ephors sent back a one word reply: “If.” Subsequently, both Philip and Alexander would avoid Sparta entirely.