Commentary for Numbers 14:16

מבלתי יכלת וגו׳ BECAUSE THE LORD IS UNABLE etc., — “He is unable to bring them into the land on Canaan because the inhabitants of the land are strong and mighty; and besides, one king, Pharaoh, is not like thirty-one kings, who inhabit Canaan”. — This is what they will say concerning the inhabitants of this land (these kings): מבלתי יכלת because He had no power to bring them into the Land He hath slaughtered them (Midrash Tanchuma, Sh'lach 13).

מבלתי יכלת, "due to an inability, etc." This may either mean that the people would believe that G'd never had such power or that He had lost such power due to old age, etc. (compare Psalms 14,1). Either way, Moses could not envisage a greater desecration of G'd's Holy Name than to allow people to arrive at such conclusions.

Now the Torah explains first what would prompt the Canaanites to say what Moses assumed they would be saying if G’d were to wipe out the Jewish nation. They would recall that they had heard about what G’d had done on behalf of the Jewish nation, how He had displayed so much fondness for them in the manner in which He had redeemed them from their slavery in Egypt. If, in spite of this, He had now killed them in one fell swoop, this was not because He suddenly hated them, but because He realised that He could not keep His promise to them, being unable to overcome the 31 kings who banded together to resist the invasion of their country. Moses appeals for G’d to reconsider so that His Holy name would not be desecrated by the nations.