Or HaChaim on Numbers 14:43/אור החיים על במדבר י״ד:מ״ג

1 כי העמלקי והכנעני שם, "for the Amalekite and the Canaanite are there, etc." The meaning is clear: The ten spies had given as the reason that the people would be unable to conquer the land the very presence of the Amalekite and the Canaanite. In view of this, any attempt to invade that part of the land knowing that G'd was not in their midst was an act of rebellion as it reflected the fact that they relied on themselves rather than on G'd. Moses warned that the certain result of such an attempt would be that they would fall by the sword just as the ten spies had predicted.