"To teach them war"

God warned Israel of the danger of entering into agreement with other peoples without overthrowing their pagan altars. How can we fulfill our "peaceful" mission today of bringing others to Christ without taking the risk of failing her completely by doing it too friendly? Let's talk about it.

And ye shall make no league with the inhabitants of this land; ye shall throw down their altars: but ye have not obeyed my voice: why have ye done this?
Wherefore I also said, I will not drive them out from before you; but they shall be {as thorns} in your sides, and their gods shall be a snare unto you.
Judges 2:2,3

And these are the nations which the LORD hath left to stand, to prove them to Israel, even to all that knew not of all the wars of Canaan.
So that the generations of the children of Israel might know them (to teach them war), at least those who knew not before.
Judges 3: 1,2