First: Islam.
Second: the jizyah.
Third: if they refuse, then fighting.
This was the basis upon which the Prophet ﷺ would dispatch military detachments. (1)
He also said: “And they may be bombarded with a catapult.” The catapult is comparable to what artillery is in our time. In earlier times, they would place the catapult between two wooden supports, with a crossbeam over them and strong ropes attached to it. A stone the size of a head or thereabouts would then be placed in a cupped sling. Strong men would pull it back and then release it; once the stone was launched, it would travel a great distance. They used it in warfare. Thus, it is permissible to bombard the disbelievers with a catapult. In the present age, catapults no longer exist, but there are things that serve the same purpose, such as aircraft, artillery, missiles, and the like.
He also said: “Even if a child and the like are killed unintentionally.” It is well known that if we bombard them with a catapult, it will destroy whomever it strikes, whether a combatant, an elderly person who does not fight, a woman, or a child. But this is not intentional, and if it is unintentional, there is no harm in it. As for deliberately bombarding children, women, and those who do not fight, that is unlawful and impermissible. However, what is permitted incidentally is not permitted independently. The Messenger ﷺ bombarded the people of al-Ta'if with a catapult (2), so the sunnah establishes this, and fighting may at times require it.