John Gill 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)

And I will bring a sword upon you War upon them by the sword of their enemies; they that use and kill with the sword, as the Targums of Onkelos and Jonathan; their neighbours that delighted in war, and bore an implacable hatred to them, and gladly embraced every opportunity of shedding their blood, and ravaging their country.

that shall avenge the quarrel of [my] covenant The covenant made with them at Sinai, which they transgressed, and for which vengeance would be taken on them in this way, God so ordering it in his providence, though the enemy meant it not, (Isaiah 10:5–7).

and when you are gathered together within your cities From the fields and villages, fleeing from the enemy invading and destroying, to their fortified towns and cities for safety.

I will send the pestilence among you Which shall destroy those that escaped the sword, and thought themselves safe in a strong city, and even the very soldiers in the garrisons, who were set for the defence of the city.

and you shall be delivered into the hand of the enemy So many being taken off by the pestilence, there would not be a sufficient number to defend the place, and therefore obliged to give it up, by which means those that escaped the pestilence would fall into the hands of the enemy.