Albert Barnes Commentary Psalms 48:4

"For, lo, the kings assembled themselves, They passed by together." — Psalms 48:4 (ASV)

For, look, the kings were assembled - There is evidently an allusion here to some fact that had occurred; some gathering of kings and their armies, intending to besiege or attack Jerusalem. The kings referred to, if the allusion here is, as is believed, to the time of Jehoshaphat, were the kings of Ammon and of Moab, and of Mount Seir, and perhaps others not specifically mentioned, who came up against Jehoshaphat (2 Chronicles 20:1), (2 Chronicles 20:10).

They passed by together - That is, they were struck with dismay; they were so impressed with the beauty, the majesty, the strength of the city, that they passed along without daring to attack it. Or, perhaps, the meaning may be that they were defeated and overthrown as suddenly as if the mere sight of the city had filled their minds with dread and had made them cease their intended assault. (Compare 2 Chronicles 20:22–25).