Charles Ellicott Commentary Leviticus 26:25

"And I will bring a sword upon you, that shall execute the vengeance of the covenant; and ye shall be gathered together within your cities: and I will send the pestilence among you; and ye shall be delivered into the hand of the enemy." — Leviticus 26:25 (ASV)

That shall avenge the quarrel of my covenant. —Better, that shall avenge my covenant, that is, the sword, which shall avenge the breach of the Divine covenant; a war, which will devastate them because of their rebellion against the covenant God. Therefore, the Chaldee Versions render it, “that shall avenge on you the vengeance because you have transgressed against the words of the law.”

And when you are gathered together within your cities. —When, completely defeated in the battlefield, the Israelites escape from the avenging sword into their fortified cities, they will then become a prey to pestilence, so that the surviving remnant will prefer to deliver themselves over into the hands of the relentless enemy. (Ezekiel 5:12; Ezekiel 7:15.)