The first time God told the Israelites to invade Canaan, He suggested that He would do the invading, and they would just go along for the ride:
Exodus 23:23 For my angel will go before you and bring you into the land of the Amorites, Hittites, Perizzites, Canaanites, Hivites, and Jebusites, so you may live there. And I will destroy them completely. …
23:27 I will send my terror ahead of you and create panic among all the people whose lands you invade. I will make all your enemies turn and run.
The Israelites had just escaped an attempted genocide by walking across the floor of the Red Sea, and then watched as this same miracle destroyed the pursuing Egyptians. They’d successfully defended an Amalekite ambush just by keeping Moses’ staff raised (Exodus 17:8-16). They would have had to have been acutely aware they were alive only by the favour of God.
Why wouldn’t God keep fighting for them?
The Israelites did have to take some responsibility, though:
Exodus 23:24 You must not worship the gods of these nations or serve them in any way or imitate their evil practices. Instead, you must utterly destroy them and smash their sacred pillars. You must serve only the Lord your God.
But at this stage, it’s not suggested that Israelites have to rush in and kill every living person. They can watch while God drives them out, then they destroy the nation (whatever that means) and destroy the idols.
This was an offer they were happy to accept.