Radak: It is inhabited lands.

Rashi (from Shocher Tov): Until the Ketz (deadline for Mashi'ach), He used to judge them with mercy, according to their straightness; He would judge them at night, when they are sleeping from (not doing) Aveiros.

Radak: Just like He judged my (David's) case b'Tzedek (justly), so may He judge at all times the nations b'Tzedek and straightly. When one nation overpowers another, this is Mishpat from Hashem.

Malbim: He will wage the battle of the oppressed from their oppressors, and not veer their law, for He judges b'Tzedek and straightly. There are three divisions. For Le'umim, He judges their Mishpat and Din. Based on this, He punishes one who agitates the land, i.e. destroyed their cities.