Adam Clarke on Daniel 7:1

In the first year of Belshazzar - This is the same Belshazzar who was slain at the taking of Babylon, as we have seen at the conclusion of chap. 5. That chapter should have followed both this and the succeeding. The reason why the fifth chapter was put in an improper place was, that all the historic parts might be together, and the prophetic be by themselves; and, accordingly, the former end with the preceding chapter, and the latter with this. The division therefore is not chronological but merely artificial.

Told the sum of the matters - That he might not forget this extraordinary dream, he wrote down the leading particulars when he arose.