Happy shall he be that taketh - Margin, as in Hebrew, rock. This refers to what was not uncommon in ancient warfare, as it is now among savage tribes - the indiscriminate slaughter of those of all ages, and of both sexes, in war. It was expressly foretold of Babylon that this would occur (see Isaiah 13:16, and the notes at that place), and there may be a reference here to that prediction, and the psalmist may mean to say that the man would be accounted happy, or would be happy, who wreaked vengeance on Babylon in carrying out that prophecy. The idea is, "This will certainly occur, for it is foretold, and happy or fortunate will he be who is the instrument in fulfilling it." Compare 2-Kings 8:12; Nahum 3:10; Hosea 13:16. See also Homer, II xxii. 63,373, following It is impossible to reconcile such barbarous customs with the idex of "honorable war," or with the principles of war as carried on among "civilized" nations now.
It should be added, however, that there is much - very much - that is practiced in war by "civilized" nations still, which it is equally impossible to reconcile with any just notions of morality or humanity, and which in coming ages, and when people shall come to view things aright, will seem to the people of those times to be not less monstrous, strange, and barbarous. In regard to this passage, we are not necessarily to suppose that the author of the psalm approved of this, or desired it, or prayed for it. He looked forward to the fulfillment of a prediction; he saw that a just and terrible judgment would certainly come upon Babylon; he expressed that in the common language of the times, and states the manner in which it would occur; he described the feelings - the gratification - of those who would execute the divine purpose in the overthrow of Babylon; he referred to the estimate in which the conqueror would be held by people, and the glory of the achievement as giving him fame among people.
It must be admitted that the feelings of the author of the psalm appear to accord with this; that he considers it proper that the city should be destroyed; and that he regards its overthrow as a righteous judgment, and as a thing to be desired in the divine administration. It is true that he might approve of such an overthrow, and see it to be right - he might describe the feelings of those by whom it would be done, their joy, their exultation, and even their barbarity, without himself approving of their barbarity, or sympathizing with their feelings, or partaking of their spirit; but still it cannot in fairness be denied that there is an apparent approval of the act here referred to, which savors more of imprecation than forgiveness, and which is apparently prompted more by the spirit of revenge than by a desire of just punishment. On this subject, however, see the General Introduction, Section 6 (4); and the notes at Psalm 109:10. A correct record may be made, whether of facts or of feelings, without any design of expressing either approbation or disapprobation on the part of the historian, the prophet, or the poet.

Happy - that taketh and dasheth thy little ones - That is, So oppressive hast thou been to all under thy domination, as to become universally hated and detested; so that those who may have the last hand in thy destruction, and the total extermination of thy inhabitants, shall be reputed happy - shall be celebrated and extolled as those who have rid the world of a curse so grievous. These prophetic declarations contain no excitement to any person or persons to commit acts of cruelty and barbarity; but are simply declarative of what would take place in the order of the retributive providence and justice of God, and the general opinion that should in consequence be expressed on the subject; therefore praying for the destruction of our enemies is totally out of the question. It should not be omitted that the Chaldee considers this Psalm a dialogue, which it thus divides: - The three first verses are supposed to have been spoken by the psalmist, By the rivers, etc. The Levites answer from the porch of the temple, in Psalm 137:4, How shall we sing, etc. The voice of the Holy Spirit responds in Psalm 137:5, Psalm 137:6, If I forget thee, etc. Michael, the prince of Jerusalem, answers in Psalm 137:7, Remember, O Lord, etc. Gabriel, the prince of Zion, then addresses the destroyer of the Babylonish nation, in Psalm 137:8, Psalm 137:9, Happy shall be he that rewardeth thee, etc. To slay all when a city was sacked, both male and female, old and young, was a common practice in ancient times. Homer describes this in words almost similar to those of the psalmist: -
Υἱας τ' ολλυμενους, ἑλκυσθεισας τε θυγατρας,
Και θαλαμους κεραΐζομενους, και νηπια τεκνα
Βαλλομενα προτι γαιῃ εν αινῃ δηΐοτητι,
Ἑλκομενας τε νυους ολοης ὑπο χερσιν Αχαιων.
Il. lib. xxii., ver. 62.
My heroes slain, my bridal bed o'erturned;
My daughters ravished, and my city burned:
My bleeding infants dashed against the floor;
These I have yet to see; perhaps yet more.
Pope.
These excesses were common in all barbarous nations, and are only prophetically declared here. He shall be reputed happy, prosperous, and highly commendable, who shall destroy Babylon.

(h) Happy [shall he be], that taketh and dasheth thy little ones against the stones.
(h) He alludes to Isaiah's prophecy in (Isaiah 13:16) promising good success to Cyrus and Darius, whom ambition moved to fight against Babylon, but God used them as his rods to punish his enemies.

Happy shall he be that taketh and dasheth thy little ones against the stones. That takes the infants from their mothers' breasts, or out of their arms, and dashes out their brains against a "rock", as the word (k) signifies; which, though it may seem a piece of cruelty, was but a just retaliation; the Babylonians having done the same to the Jewish children, and is foretold elsewhere should be done to theirs, Isaiah 13:16. Nor is this desired from a spirit of revenge, but for the glory of divine justice, and that such a generation of cruel creatures might be rooted out of the earth; see Revelation 2:2. Some allegorically understand this of crushing and mortifying the first motions of sin in the heart; but such a sense seems to have no place here.
(k) "ad petram", V. L. Pagninus, Montanus, &c. "ad repem", Cocceius.