In truth, the allegation springs from both causes at once: ignorance of Islam and its history, and sectarian hostility toward it. For Islam was founded upon calling people with wisdom and good counsel; it proclaimed peace and invited to it—for the very word salām (peace) is derived from Islām.
Whoever surveys the Qur’an and the purified Sunnah—with the Prophet’s counsels to his field commanders and his own conduct in battle—will realize that Islam came with wisdom, mercy, peace, and concord; it came to set things right, not to sow corruption.
Consider the Almighty’s words: “There is no compulsion in religion; right guidance has become distinct from error” (Qur’an 2:256). Consider also: “Had your Lord willed, everyone on earth would have believed, all of them together; so will you compel people until they become believers?” (Qur’an 10:99).
And reflect on His statement: “Allah does not forbid you from showing kindness and acting justly toward those who do not fight you on account of religion and do not drive you from your homes; surely Allah loves those who act justly” (Qur’an 60:8); and His command: “Fight in the path of Allah those who fight you, but do not transgress” (Qur’an 2:190). Many other verses convey the same meaning.
As for the Sunnah, every military action of the Prophet ﷺ and every directive he issued to his commanders testifies to this principle. In the hadith of Buraydah recorded by Muslim, whenever he appointed a leader over a detachment or an army he would first counsel him to fear Allah privately, then to treat the Muslims with goodness. He would go on to say:
“Advance in the name of Allah, in the path of Allah, against those who deny Allah. Do not embezzle (ghulūl), do not act treacherously, do not mutilate, and do not kill an infant.”
He ﷺ also forbade “the killing of women and children” (agreed upon by Bukhārī and Muslim).
Another narration states: “Set out in the name of Allah. Fight in Allah’s path those who disbelieve in Him. Do not act treacherously, do not embezzle, do not mutilate, and do not kill children or the inhabitants of monasteries.” He added: “And do not kill an elderly man who is decrepit.”
Abū Bakr al-Ṣiddīq, when dispatching Yazīd ibn Abī Sufyān as commander over one quarter of the Syrian front, instructed him:
“I charge you with ten things: 1) Do not kill a woman; 2) nor a child; 3) nor an aged, infirm man; 4) do not cut down a fruit-bearing tree; 5) do not demolish an inhabited place; 6) do not slaughter a sheep except for food.”