The most influential tactic off spreading Islam in the early stages was through force of the conversion or death. As Quran states in these Surah's we can see how much Islam would be the religion of peace:
Quran (2:191-193) - "And slay them wherever ye find them, and drive them out of the places whence they drove you out, for persecution [of Muslims] is worse than slaughter [of non-believers]... but if they desist, then lo! Allah is forgiving and merciful. And fight them until persecution is no more, and religion is for Allah."
Quran (2:216) - "Fighting is prescribed for you, and ye dislike it. But it is possible that ye dislike a thing which is good for you, and that ye love a thing which is bad for you. But Allah knoweth, and ye know not."
Quran (4:74) - "Let those fight in the way of Allah who sell the life of this world for the other. Whoso fighteth in the way of Allah, be he slain or be he victorious, on him We shall bestow a vast reward."
Quran (4:76) - "Those who believe fight in the cause of Allah…"
Quran (4:89) - "They but wish that ye should reject Faith, as they do, and thus be on the same footing (as they): But take not friends from their ranks until they flee in the way of Allah (From what is forbidden). But if they turn renegades, seize them and slay them wherever ye find them; and (in any case) take no friends or helpers from their ranks."
Quran (5:33) - "The punishment of those who wage war against Allah and His messenger and strive to make mischief in the land is only this, that they should be murdered or crucified or their hands and their feet should be cut off on opposite sides or they should be imprisoned; this shall be as a disgrace for them in this world, and in the hereafter they shall have a grievous chastisement"
Quran (8:12) - "I will cast terror into the hearts of those who disbelieve. Therefore strike off their heads and strike off every fingertip of them"
Quran (8:39) - "And fight with them until there is no more fitna (disorder, unbelief) and religion should be only for Allah"
Quran (8:65) - "O Prophet, exhort the believers to fight..."
Quran (9:14) - "Fight them, Allah will punish them by your hands and bring them to disgrace..."
Quran (9:29) - "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."
Quran (9:41) - "Go forth, light-armed and heavy-armed, and strive with your wealth and your lives in the way of Allah! That is best for you if ye but knew."
Quran (9:73) - "O Prophet! strive hard against the unbelievers and the hypocrites and be unyielding to them; and their abode is hell, and evil is the destination."
Quran (9:88) - "But the Messenger, and those who believe with him, strive and fight with their wealth and their persons: for them are (all) good things: and it is they who will prosper."
Quran (9:123) - "O you who believe! fight those of the unbelievers who are near to you and let them find in you hardness."
Quran (17:16) - "And when We wish to destroy a town, We send Our commandment to the people of it who lead easy lives, but they transgress therein; thus the word proves true against it, so We destroy it with utter destruction."
Quran (25:52) - "Therefore listen not to the Unbelievers, but strive against them with the utmost strenuousness..."
Quran (47:3-4) - "Those who reject Allah follow vanities, while those who believe follow the truth from their lord. Thus does Allah set forth form men their lessons by similitude. Therefore when you meet in battle those who disbelieve, then smite the necks until when you have overcome them, then make (them) prisoners,"
Quran (66:9) - "O Prophet! Strive against the disbelievers and the hypocrites, and be stern with them. Hell will be their home, a hapless journey's end."
Hard to believe there are even more Surah's like above examples.
One of the good examples of this barbaric and cruel behavior was the conquest of the great Persian Empire which at the time if not the most advanced in art, culture, science and more certainly was one of the best.
Arabs first entered Sassanid territory in 633, when General Khalid ibn Walid invaded what is now Iraq.
After the Treaty of Hudaybiyyah in 628, Islamic tradition holds that Muhammad sent many letters to the princes, kings, and chiefs of the various tribes and kingdoms of the time, inviting them to convert to Islam. These letters were carried by ambassadors to Persia, Byzantium, Ethiopia, Egypt, Yemen on the same day.
With regards to Persia, Muslim histories further recount that at the beginning of the seventh year of migration, Muhammad appointed one of his officers, Abdullah Huzafah Sahmi Qarashi, to carry his letter to Khosrau II inviting him to convert:
"In the name of Allah, the Beneficent, the Merciful. From Muhammad, the Messenger of Allah, to the great Kisra of Iran. Peace be upon him, who seeks truth and expresses belief in Allah and in His Prophet and testifies that there is no god but Allah and that He has no partner, and who believes that Muhammad is His servant and Prophet. Under the Command of Allah, I invite you to Him. He has sent me for the guidance of all people so that I may warn them all of his wrath and may present the unbelievers with an ultimatum. Embrace Islam so that you may remain safe. And if you refuse to accept Islam, you will be responsible for the sins of the Magi."
The Sassanid court ceremony was notoriously intricate, and it is unlikely that a letter from what at the time was a minor regional power would have reached the hands of the Shahanshah until years latter the copy of communications between Sassanian Yazdgird III (632 AD - 651 AD) and Omar Ibn Al Khatab Khalifat Al Muslemin, after the battle of Ghadesiyeh. The original copy of this letter from Yazdgird III (632 AD - 651 AD) is kept in the British museum of London. The date in which this letter has been written is after the battle of Ghadesiyeh.
From: Omar Ibn Al Khatab Khalifat Al Muslemin
To: Yazdgird III Shahanshah of Persian Empire
Bism-ellah Ar'rahman Ar'rhim To the Shah of the Fars
I do not foresee a good future for you and your nation save your acceptance of my terms and your submission to me. There was a time when your country ruled half the world, but see how now your sun has set. On all fronts your armies have been defeated and your nation is condemned to extinction. I point out to you the path whereby you might escape this fate. Namely, that you begin worshipping the one god, the unique deity, the only god who created all that is. I bring you his message. Order your nation to cease the false worship of fire and to join us, that they may join the truth.
Worship Allah the creator of the world. Worship Allah and accept Islam as the path of salvation. End now your polytheistic ways and become Muslims that you may accept Allah-u-Akbar as your savior. This is the only way of securing your own survival and the peace of your Persians. You will do this if you know what is good for you and for your Persians. Submission is your only option. Allah u Akbar.
The Caliph of Muslims Omar Ibn-Khat'tab
Response of the Persian King:
In the name of Ahura Mazda, creator of Life and Intelligence you in your letter wrote that you want to direct us towards your God, Allah, without having the true knowledge of who we are and what do we worship! It is amazing that you occupy the position of Caliph (Ruler) of Arabs, yet your knowledge is the same as a lowly Arab rambler, roaming in deserts of Arabia and same as a desert tribal man (mardak) you offer me to worship a united and single God without knowing that it has been thousands of years that Persians worship the mono God and they pray to him Five Times a day! In this land of culture and art this has been the normal path of life for years. When we established the tradition of hospitality and good deeds in the world and we waved the flag of "Good Thoughts, Good Words, Good Deeds" in our hands, you and your ancestors were roaming the deserts, eating Lizards for you had nothing else to feed yourselves and burying your innocent daughters alive (an old Arab tradition because they preferred male children to female). Tazi people have no value for God's creatures! You behead God's children, even the POWs (Prisoners of War), Rape Women, bury your daughters alive, attack the Caravans, mass murder, kidnap people's wives and steal their property! Your hearts are made of stone; we condemn all these Evil which you do. How can you teach us Godly Ways when you commit these actions? You tell me to stop my Fire Worship! We, Persians see the Love of Creator and power of inventor in the light of Sun and warmth of Fire. Lights and Warmth of the Sun and Fire makes us see the light of truth and warms our hearts to the creator and to one another. It helps us to be kind to one another; it enlightens us and makes us to keep Mazda's Flame, alive in our hearts. Our lord is Ahura Mazda and it is strange that you people also, just discovered him and named him Allah.
But we are not the same as you; we are not in the same level as you. We help other human being, we spread love among humanity, we spread Good throughout the Earth, we have been spreading our culture but in respect for other cultures throughout the whole world for thousands of years, yet you in the name of Allah invade other men's land! You mass murder the people, create famine, fear and poverty for others, you create Evil in the name of Allah. Who is responsible for this entire catastrophe? Is it Allah who commands you to murder, pillage and to destroy? Is it you the followers of Allah who do this in his name? Or is it both? You have risen from heat of the deserts and burnt out infertile lands with no resources, you want to teach people the love of God by your military campaigns and the power of your Swords! You are Desert Savages, yet you want to teach urban people like us who lived in the cities for thousands of years, the love of God! We have thousands of years of culture behind us, a powerful tool indeed!
Tell us? With all your military campaigns, barbarianism, murder and pillage in the name of Allah, what have you taught to this Muslim Army? What knowledge have you taught the Muslim that you also insist on teaching it to non Muslim? What culture have you learned from our Allah, now that you want to force-teach it to others? Alas, ...... that today our Persian Armies of Ahura have been defeated from your recently Allah Worshiping Armies; Now, our people have to worship the same God, the same Five times a day, but forced by the sword to call him Allah and pray to him in Arabic because your Allah only understands Arabic I suggest, you and your gang of bandits pack up and move back to your deserts where they are used to live. Take them back where they used to the burning heat of the sun, tribal life, eating Lizards and drinking Camel Milk. I forbid you to let your band of thieves loose in our fertile lands, civilized cities and our glorious nation. Don't turn these "beasts with hearts of stone" loose, to mass murder our people, kidnap our women and children, rape our wives and send our daughters to Mecca as slaves! Don't let them do these crimes in the name of Allah put a stop to your criminal behavior. Aryans are forgiving, warm, hospitable, and decent people and everywhere they went, they have spread seeds of friendship, love, knowledge and truth; therefore, they shall not punish you and your people for your pirate ways and criminal acts. I beg you to remain with your Allah in your deserts and do not move close to our civilized cities, for your believers are "Much Fearful" and your behavior is "Most Barbaric."
The attack of the Islamic Arabs led to the end of the Sassanid Empire in 644, the fall of Sassanid dynasty in 651 and the eventual decline of the Zoroastrian religion in Iran.
The most astonishing fact is even though Iranians were forced to convert to Islam at the time, still the majority of Iranians are now Muslim and they name their children after the so called holly men or heroes who destroyed their heritage, culture, empire and their bright future.