war cry

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A cry uttered by combatants as they attack; a battle cry.
• noun A phrase or slogan used to rally people to a cause.

from The Century Dictionary.

• noun A cry or phrase used in war for mutual recognition or encouragement; a short pithy expression used in common by a body of troops in charging an enemy: as, “Saint George!” was the war-cry of England, “Montjoie Saint Denis!” the war-cry of France.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An exclamation intended to rally soldiers in battle.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a slogan used to rally support for a cause
• noun a yell intended to rally a group of soldiers in battle