This is an interesting little post about the phrase Allahu Akbar. OK, so it's often used simply as an everday expression of joy, it's an integral part of prayers in Islamic countries.
It can also be a war cry: for Allahu Akbar (or Allahu Akhbar sometimes) literally means "God is Great". But it's aslo much more than that, for Allahu Akbar can mean:
Allahu Akbar can also be used as a fast and colloquial manner of converting to Islam. The simple repetition of the phrase three times, "Allahu Akbar, Allahu Akbar, Allahu Akbar" is taken by most Islamic jurists as being sufficient expression of the core Islamic beliefs that those who say it are now Muslim. Traditionally (and perhaps anecdotally) those who say this either in battle or afterwards when captured will be deemed to have converted, become part of the Ummah (the Islamic community) and thus no longer be considered as enemies.