Charles Ellicott Commentary 1 Chronicles 12:37

"And on the other side of the Jordan, of the Reubenites, and the Gadites, and of the half-tribe of Manasseh, with all manner of instruments of war for the battle, a hundred and twenty thousand." — 1 Chronicles 12:37 (ASV)

On the other side. — Better, from the other side; that is, from Perea.

With all manner of instruments of war for the battle. — With all kinds of weapons of war-like service. The large total of 120,000 for the two and a half Eastern tribes is certainly remarkable. But, admitting the possibility of corruption in the numbers here and elsewhere, the lack of other documents with which the text might be compared renders further criticism superfluous.