The theme of these five verses is that the Lord does not approve of the military alliances which the kingdoms of Israel and Judah are making. The Lord has taught them in the past to trust in Him. He has even fought their battles. In a battle between the children of Israel and the Amorites ’the Lord discomfited them before Israel, and slew them with a great slaughter at Gibeon…the Lord cast down great stones from heaven upon them unto Azekah, and they died: they were more which died with hailstones than they whom the children of Israel slew with the sword…And Joshua said unto to them, Fear not, nor be dismayed, be strong and of good courage: for thus shall the Lord do to all your enemies against whom he fight’ (Joshua 10:10-11,25).
If the children of Israel had continued to worship the Lord, He would have continued to fight their battles in just as dramatic a fashion. When the Lord is fighting your battles, you do not need the help of neighboring nations. Nevertheless, the kingdom of Israel had made an alliance with Syria, and the kingdom of Judah had made an alliance with Assyria. They had begun to put their trust in the arm of flesh instead of the Lord. Hence the counsel, ’Sanctify the Lord of Hosts himself, and let him be your fear and let him be your dread’ (v. 13).