Of course, a whole lot of people were already living in the Promised Land when Moses showed up with the Children of Israel. It was good land.
According to Moses, GOD had two sets of rules for warfare with cities, depending on whether the target cities were outside the Promised Land (“very far from you”), or inside it (“cities of the nations here”).
For the first ones, GOD says, offer them peace as slaves. If they accept, enslave them. If they don’t accept, then besiege the city, capture it, kill all the males, and keep the women and cattle as booty. You can afford to be this generous, because it’s outside the Promised Land.
But if the city is inside the Promised Land, you have to kill everything – men, women, children, animals – “in order to prevent them infecting you with their immoral practices”. I kid you not.
Here are the rules, from Deuteronomy 20, verses 10-15 for the first lot, 16-18 for the second.
 When you draw near to a city to fight against it, offer terms of peace to it.
 And if its answer to you is peace and it opens to you, then all the people who are found in it shall do forced labor for you and shall serve you.
 But if it makes no peace with you, but makes war against you, then you shall besiege it;
 and when the LORD your God gives it into your hand you shall put all its males to the sword,
 but the women and the little ones, the cattle, and everything else in the city, all its spoil, you shall take as booty for yourselves; and you shall enjoy the spoil of your enemies, which the LORD your God has given you.
 Thus you shall do to all the cities which are very far from you, which are not cities of the nations here.
 But in the cities of these peoples that the LORD your God gives you for an inheritance, you shall save alive nothing that breathes,
 but you shall utterly destroy them, the Hittites and the Amorites, the Canaanites and the Per’izzites, the Hivites and the Jeb’usites, as the LORD your God has commanded;
 that they may not teach you to do according to all their abominable practices which they have done in the service of their gods, and so to sin against the LORD your God.
Having got control of Palestine for 1500 years through genocide and ethnic cleansing, religious Jews (including Jesus, obviously) were in no mood to have the idolatrous, polytheist, pig-eating Romans come in and run the country. Their duty was to wipe the Romans out.
Unfortunately that thinking is still alive today among religious Jews. Even if Palestinians are neither idolatrous nor polytheist nor pig-eating, the more fanatical among the Jews do not see them as worshiping the same God, and want to wipe them out. But God is said to have promised the land to the descendants of Abraham – and Arabs, too, claim descent from Abraham. So the promise of the Covenant is fulfilled if Jews or Arabs live in the Promised Land… from the river of Egypt to the Euphrates.