John Gill Commentary Ezra 4:16

"We certify the king that, if this city be builded, and the walls finished, by this means thou shalt have no portion beyond the River." — Ezra 4:16 (ASV)

We certify the king, that if this city be built again, and the walls of it set up As it formerly was, and now attempted, as they suggest:

by this means you will have no portion on this side the river ; the river Euphrates; intimating that the Jews would not only shake off his yoke, and refuse to pay tribute themselves, but would seize on all his dominions on that side the river, and annex them to their own.