Narrated Anas bin Malik: Allah's Messenger (ﷺ) said, "I have been ordered to fight the people till they say: 'None has the right to be worshipped but Allah.' And if they say so, pray like our prayers, face our Qibla and slaughter as we slaughter, then their blood and property will be sacred to us and we will not interfere with them except legally and their reckoning will be with Allah."
Fight those who do not believe in Allah or in the Last Day and who do not consider unlawful what Allah and His Messenger have made unlawful and who do not adopt the religion of truth from those who were given the Scripture - [fight] until they give the jizyah willingly while they are humbled.
So whoever deviates from the Book is to be brought back with iron, i.e. by force. Hence the soundness of the religion is based on the Qur’aan and the Sword.
If Islam was only spread by peaceful means, what would the kuffaar have to be afraid of? Of mere words spoken on the tongue?
Yes, it is the religion of peace but in the sense of saving all of mankind from worshipping anything other than Allaah and submitting all of mankind to the rule of Allaah.
It's right there in their doctrine. The world already belongs to Allah, and the Muslims have an obligation to subordinate the world to Islam, by force.
Source: twitter.com