When King Agesilaos was asked why his swords were so short:
"Because we fight close to the enemy."

Why losing your sheild in phalanx position was worse than losing anything else:
"Because the latter they put on for thier own protection, but the shield for the common good of the whole line."

Aeginetan to Lampon after the victory at Platea 479 BC:
"When Leonides was killed at Thermopylae, Mardonius and Xerxes had him decapitated and his head stuck on a pole. If you now pay them back in kind, you will win the applause of not only Spartans but the whole of Greece. Impale Mardonius's body, and then Leonidas, your father's brother, will be avenged."

Sun Tzu:
"So in war, the way is to avoid what is strong and strike at what is weak"

A personal fravorite of mine from a man called Gais Julius Caesar on the crossing of the Rubicon:
"iacta alea est"
"The die is cast"
Another from the same man would be:
"Veni, Vidi, Vici"

Try and figure that one out ;)