On the authority of the son of 'Umar (may Allah be pleased with them both) that the Messenger of Allah* said:
I have been ordered to fight against people until they testify that there is no god but Allah and that Muhammad is the Messenger of Allah and until they perform the prayers and pay the zakat, and if they do so they will have gained protection from me for their lives and property, unless (they do acts that are punishable) in accordance with Islam, and their reckoning will be with Allah the Almighty.
Muhammad* meant that he must fight those who either wage war against Islam, prevent the peaceful spreading and preaching of Islam or apostates. The Quran says "Let there be no compulsion in religion", Islam doesn't preach forced conversion and has always remained the most tolerant of faiths.