John Gill Commentary Numbers 14:43

"For there the Amalekite and the Canaanite are before you, and ye shall fall by the sword: because ye are turned back from following Jehovah, therefore Jehovah will not be with you." — Numbers 14:43 (ASV)

For the Amalekites and the Canaanites [are] there before you ,
&c.] Having removed from the valley, (Numbers 14:25) ; or else had detached a party to defend the pass on the top of the mountain, and where perhaps they designed to feign a retreat if they found it proper, and draw them into a combat in the valley:

and you shall fall by the sword :
by the sword of the Amalekites and Canaanites:

because you are turned away from the Lord :
from the word of the Lord, from hearkening to and obeying his command:

therefore the Lord will not be with you ;
the consequence of which must be bad for them.