"But when the forbidden months are past, then fight and slay the Pagans wherever ye find them, and seize them, beleaguer them, and lie in wait for them in every stratagem (of war); but if they repent, and establish regular prayers and practice regular charity, then open the way for them: for Allah is Oft-forgiving, Most Merciful." (Surah 9:5)
"O ye who believe! Fight those of the disbelievers who are near to you, and let them find harshness in you, and know that Allah is with those who keep their duty (unto Him). " (Surah 9:123)
"Fight those who believe not in Allah nor the Last Day, nor hold that forbidden which hath been forbidden by Allah and His Messenger, nor acknowledge the religion of Truth, (even if they are) of the People of the Book, until they pay the Jizya with willing submission, and feel themselves subdued." (Surah 9:29)
"In the Muslim community, the holy war is a religious duty, because of the universalism of the Muslim mission and (the obligation to) convert everybody to Islam either by persuasion or by force. Therefore, caliphate and royal authority are united in Islam, so that the person in charge can devote the available strength to both of them at the same." --Ibn Khaldun
"'The Prophet said, "The person who participates in (holy battles) in Allah's cause and nothing compels him to do so except belief in Allah and His Apostles, will be recompensed by Allah either with a reward or booty (if he survives) or will be admitted to Paradise (if he is killed in the battle as a martyr)."
--Sahih al-Bukhari Vol. 1, Bk. 2, No. 35