Commentary for Numbers 14:43

כי על כן שבתם — means: for (כי) this will come upon you, because (על כן) ye have turned away etc. (cf. Rashi on Genesis 18:5 and Numbers 10:31).

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.

כי על כן שבתם, the line has the word אשר missing; we find a similar construction with the missing word אשר in Genesis 39 4 where the Torah writes וכל יש לו בידו instead of וכל אשר יש לו בידו.