[Chani]

In Islam, an invitation to the religion is the first step in a declaration of war.

Yes it seems I read/heard that they must first offer an invitation for the enemy to convert..and if they don't, then the heads can begin to fly.

[Eric in the Ozarks]

The Turks did this prior to their invasion of Persia. Giving the opponent an opportunity to convert is the required "warning" from the Koran.