Or HaChaim on Numbers 33:55/אור החיים על במדבר ל״ג:נ״ה

1 ואם לא תורישו את יושבי הארץ, "And if you will fail to drive out the inhabitants of the land, etc." According to Megillah 10 which says that wherever the word והיה occurs it alludes to something joyful, the meaning of the word here would be that although the reason the Israelites would decide to allow some of these inhabitants to remain would be in order that they perform menial duties for them, etc., i.e. in order to enjoy their services, G'd would see to it that instead these remaining inhabitants would prove to be thorns in their eyes and pricks in their sides and that these foreigners would harass the new owners.

2וצררו אתכם על הארץ, "and they shall harass you in the land, etc." The Torah means that not only will they harass you in the part of the land that you did not drive them out of, but they will harass you even in the parts of the country which you have conquered and driven them out of and which you have settled. They will challenge you to leave what they consider to be their country. The meaning of the word על in this context is similar to Numbers 20,24: על אשר מריתם את פי, "because you have rebelled against My word." We have a similar meaning of the word על in Deut. 32,51.