John Gill Commentary Deuteronomy 1:42

"And Jehovah said unto me, Say unto them, Go not up, neither fight; for I am not among you; lest ye be smitten before your enemies." — Deuteronomy 1:42 (ASV)

And the Lord said to me
When the people had armed themselves, and were in motion, or ready to set forward to ascend the hill:

say to them, go not up, neither fight ;
neither go up the hill, and if they did, contrary to this order, and should meet with enemies, not fight them, but retreat:

for I am not among you :
the ark of the covenant, the symbol of his presence, was then among them, but it did not go with them, it continued in the camp, (Numbers 14:44) nor did the Lord exert his power, or show himself present with them, or to be on their side, but left them to themselves, and to their enemies:

lest you be smitten before your enemies ;
God not being with them to fight for them, protect and defend them, and give them victory.