Because it's important to keep in mind the insidious nature of Islam's core beliefs with respect to the non-Islamic world, I am lifting the quote below from the web page of Sheikh Muhammad Saleh al-Munajjid, a Saudi Wahhabi cleric discussing the nature and need for jihad. This is small piece of a recent exposition on the subject, compliments of Jihad Watch. The sheikh's website is quoting from a classic, mainstream Islamic text which is discussing the onset of the Arab armed invasion of Sasanian Persia in the 630's C.E. The "mushrikeen" are the non-Muslims who are considered idolators by the Muslims, e.g., the Zoroastrians (Persian fire-worshippers) in this case. Chosroes was the Persian emperor who received Mohammad's letter "inviting" him to embrace Islam or pay the penalty. Effectively, the last one, although he was succeeded by his ill-fated grandson Yazdgard in 628. Jizyah is the money you pay to the Muslims in return for being allowed to keep your own religious identity (and your life).
This purpose of jihad was present in the minds of the Sahaabah (may Allaah be pleased with them) during their battles with the enemies of Allaah. Al-Bukhaari (2925) narrated that Jubayr ibn Hayyah said: ‘Umar sent people to all the regions to fight the mushrikeen… so ‘Umar recruited us and appointed al-Nu’maan ibn Muqarrin to lead us. When we were in the land of the enemy, the representative of Chosroes (actually Yazdgard III, n.b.) came out to us with forty thousand troops. An interpreter stood up and said: “Let one of you speak to me.” Al-Mugheerah said: “Ask whatever you want.” He asked, “Who are you?” He (al-Mugheerah) said: “We are some people from among the Arabs. We used to lead a harsh and miserable life, sucking on animal skins and date stones because of hunger, wearing clothes made of camel and goat hair, worshipping trees and rocks. While we were in this state, the Lord of the heavens and the earth, exalted be His name and glorified be His greatness, sent to us a Prophet from amongst ourselves, whose father and mother we know. Our Prophet, the Messenger of our Lord, (peace and blessings of Allaah be upon him), commanded us to fight you until you worship Allaah alone or pay the jizyah. Our Prophet (peace and blessings of Allaah be upon him) told us the message from our Lord, that whoever among us is killed will go to Paradise to enjoy delights such as no one has ever seen, and whoever among us is left will become your master.”
This is the truth that the Sahaabah and leaders of the Muslims proclaimed in their military campaigns.
For those who need a reminder that there are still devoted and pious Muslims acting on the jihad imperative, and that they are the ones who decide what is and what is not Islamic, I offer you the photo to the left. "Composition with pistol and Quran"