Jamieson, Fausset and Brown on Nahum 1:2

2. jealous--In this there is sternness, yet tender affection. We are jealous only of those we love: a husband, of a wife; a king, of his subjects' loyalty. God is jealous of men because He loves them. God will not bear a rival in His claims on them. His burning jealousy for His own wounded honor and their love, as much as His justice, accounts for all His fearful judgments: the flood, the destruction of Jerusalem, that of Nineveh. His jealousy will not admit of His friends being oppressed, and their enemies flourishing (compare Ex 20:5; 1Co 16:22; 2Co 11:2). Burning zeal enters into the idea in "jealous" here (compare Nu 25:11, 13; 1Ki 19:10).

the Lord revengeth ... Lord revengeth--The repetition of the incommunicable name Jehovah, and of His revenging, gives an awful solemnity to the introduction.

furious--literally, "a master of fury." So a master of the tongue, that is, "eloquent." "One who, if He pleases, can most readily give effect to His fury" [Grotius]. Nahum has in view the provocation to fury given to God by the Assyrians, after having carried away the ten tribes, now proceeding to invade Judea under Hezekiah.

reserveth wrath for his enemies--reserves it against His own appointed time (2Pe 2:9). After long waiting for their repentance in vain, at length punishing them. A wrong estimate of Jehovah is formed from His suspending punishment: it is not that He is insensible or dilatory, but He reserves wrath for His own fit time. In the case of the penitent, He does not reserve or retain His anger (Psa 103:9; Jr 3:5, 12; Mi 7:18).