Fight those who do not believe in Allah ( i.e. Non-Muslims) , nor in the latter day, nor do they prohibit what Allah and His Messenger have prohibited, nor follow the religion of truth (i.e. Islam), out of those who have been given the Book ( including Christians and Jews ), until they pay the tax in acknowledgment of superiority (of Islam and Muslims) and they are in a state of subjection.
(Koranic Verse: 9.29; SOURCE)
Earlier Muslims covered the cross on the church with gunny bags ( here ).
But Sharia has to be enforced completely and every Non-Muslim has to abide by it; it is only then that Muslims are at peace. It is only then that Muslims feel that there is no aggression on them. Because mere presence of disbelief in Islam becomes aggression (Koran says this in verses 2:193 and 8:39) on Islam and Muslims. Now one can see how, when Muslims say their wars are always fought in defense.
