Deuteronomy 1:42
And the LORD said unto me, Say unto them, Go not up, neither fight; for I am not among you; lest ye be smitten before your enemies. And the LORD said unto me, Say unto them, Go not up, neither fight; for I am not among you; lest ye be smitten before your enemies.
The command was to not go up and not fight, because the divine presence was not among the people, with the warning that to advance in disobedience would mean being struck down before their enemies. After rebellion the presence and help were withdrawn from Israel, so going forward against direction brought defeat.
This warning explains why the adult army that came out of Egypt was refused entrance into the promised land, for in rebellion they had not wholly followed the Lord. They went in direct opposition to the command, going out without Moses as the appointed leader, without the cloudy pillar as the symbol of the divine presence, and without the ark, and they were routed with thousands slain. By contrast, to go forward in obedience means having Moses present to strengthen hearts with hope and faith, having the Son of God enshrined in the cloudy pillar leading the way, and having the sacred ark accompanying the host.
The promises are upon condition of obedience, so obedience was necessary for the promise to be fulfilled. Success depends on the divine presence through obedience to the requirements, and those who move forward in the path of obedience humbly trusting will see formidable difficulties vanish as God works for them. The chapter stands as instruction for later generations, warning them to follow the counsel given rather than their own judgment.