In that night was Belshazzar the king of the Chaldeans slain.

Jamieson-Fausset-Brown Bible Commentary

30. Herodotus and Xenophon confirm Daniel as to the suddenness of the event. Cyrus diverted the Euphrates into a new channel and, guided by two deserters, marched by the dry bed into the city, while the Babylonians were carousing at an annual feast to the gods. See also Isa 21:5; 44:27; Jer 50:38, 39; 51:36. As to Belshazzar's being slain, compare Isa 14:18-20; 21:2-9; Jer 50:29-35; 51:57.

Matthew Henry's Concise Commentary

5:18-31 Daniel reads Belshazzar's doom. He had not taken warning by the judgments upon Nebuchadnezzar. And he had insulted God. Sinners are pleased with gods that neither see, nor hear, nor know; but they will be judged by One to whom all things are open. Daniel reads the sentence written on the wall. All this may well be applied to the doom of every sinner. At death, the sinner's days are numbered and finished; after death is the judgment, when he will be weighed in the balance, and found wanting; and after judgment the sinner will be cut asunder, and given as a prey to the devil and his angels. While these things were passing in the palace, it is considered that the army of Cyrus entered the city; and when Belshazzar was slain, a general submission followed. Soon will every impenitent sinner find the writing of God's word brought to pass upon him, whether he is weighed in the balance of the law as a self-righteous Pharisee, or in that of the gospel as a painted hypocrite.

Matthew Henry's Whole Bible Commentary

Verses 30-31

Here is, 1. The death of the king. Reason enough he had to tremble, for he was just falling into the hands of the king of terrors, v. 30. In that night, when his heart was merry with wine, the besiegers broke into the city, aimed at the palace; there they found the king, and gave him his death's wound. He could not find any place so secret as to conceal him, or so strong as to protect him. Heathen writers speak of Cyrus's taking Babylon by surprise, with the assistance of two deserters that showed him the best way into the city. And it was foretold what a consternation it would be to the court, Jer. 51:11, 39. Note, Death comes as a snare upon those whose hearts are overcharged with surfeiting and drunkenness. 2. The transferring of the kingdom into other hands. From the head of gold we now descend to the breast and arms of silver. Darius the Mede took the kingdom in partnership with, and by the consent of, Cyrus, who had conquered it, v. 31. They were partners in war and conquest, and so they were in dominion, ch. 6:28. Notice is taken of his age, that he was now sixty-two years old, for which reason Cyrus, who was his nephew, gave him the precedency. Some observe that being now sixty-two years old, in the last year of the captivity, he was born in the eighth year of it, and that was the year when Jeconiah was carried captive and all the nobles, etc. See 2 Ki. 24:13-15. Just at that time when the most fatal stroke was given was a prince born that in process of time should avenge Jerusalem upon Babylon, and heal the wound that was now given. Thus deep are the counsels of God concerning his people, thus kind are his designs towards them.