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.

Because you turned. The meaning of על כן (for) is “since” [rather than “therefore”], which Rashi explains as על אשר (because). Given that it would be incorrect to say הואיל ושבתם (since you turned away) the Torah adds the formation “verb … על כן” to the word כי which indicates causation — “since you did so…” referring to you turning away from following Hashem.

V. 43. כי על כן (siehe Bereschit 18, 5). Denn nur darum sage ich euch, dass ihr nicht euch hinauf wagen sollt, weil ihr zurückgewichen seid von der Nachfolge Gottes.

כי על כן שבתם, “for you turned away from Hashem again.” The reason why the spies frightened you was in order to turn you away from G-d, and now when they succeeded you wanted to do this once more? Do you still not place your confidence in Him? The only reason why He is not going to be on your side is that you constantly rebel against Him! If you will again transgress against His commandment you will surely be defeated until the forty years He decreed will have passed.