Principle number 23 of Basic Elements of Islam: Islam must always be defended. This theme runs throughout the Quran and the Hadith. The world is divided into Muslims and non-Muslims, and Muslims are brothers to each other. Non-Muslims are the enemy or a second-class citizen. No matter what other conflict you have with a fellow Muslim, if he is being attacked, you must defend your fellow Muslim. If Islam is being attacked, you must defend Islam no matter what.
This idea is a primary linchpin that gives justification for war with almost anybody, as you'll see in the next principle. After the enemy is defeated, of course, Muslims must establish an Islamic state.