Adam Clarke on Isaiah 47:9

These two things shall come to thee in a moment - That is, suddenly. Belshazzar was slain; thus the city became metaphorically a widow, the husband - the governor of it, being slain. In the time in which the king was slain, the Medes and Persians took the city, and slew many of its inhabitants, see Dan 5:30, Dan 5:31. When Darius took the city, he is said to have crucified three thousand of its principal inhabitants.

In their perfection "On a sudden" - Instead of בתמם bethummam, "in their perfection," as our translation renders it, the Septuagint and Syriac read, in the copies from which they translated, פתאם pithom, suddenly; parallel to רגע rega, in a moment, in the preceding alternate member of the sentence. The concurrent testimony of the Septuagint and Syriac, favored by the context, may be safely opposed to the authority of the present text.

For the multitude "Notwithstanding the multitude" - ברב berob. For this sense of the particle ב beth, see Num 14:11.