battle cry

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

• noun A rallying cry uttered in combat, especially while attacking.
• noun A slogan used by the proponents of a cause.

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

• noun military An exclamation troops yell out when going to war or battle.
• noun idiomatic By extension, a strong motto or purpose statement, especially in regards to winning a goal in sports, games or work.

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