It seems so harsh, but we don't have God's perspective. He asks for obedience and he does keep his promises. Whether we obey or not.

Numbers 33:51-56
“Speak to the children of Israel, and say to them: ‘When you have crossed the Jordan into the land of Canaan,
then you shall drive out all the inhabitants of the land from before you, destroy all their engraved stones, destroy all their molded images, and demolish all their high places;

you shall dispossess the inhabitants of the land and dwell in it, for I have given you the land to possess. 
And you shall divide the land by lot as an inheritance among your families; to the larger you shall give a larger inheritance, and to the smaller you shall give a smaller inheritance; there everyone’s inheritance shall be whatever falls to him by lot. You shall inherit according to the tribes of your fathers.
But if you do not drive out the inhabitants of the land from before you, then it shall be that those whom you let remain shall be irritants in your eyes and thorns in your sides, and they shall harass you in the land where you dwell.
Moreover it shall be that I will do to you as I thought to do to them.’”
http://www.biblegateway.com/passage/?search=numbers33:51-56&version=NKJV