John Gill Commentary 2 Kings 18:7

"And Jehovah was with him; whithersoever he went forth he prospered: and he rebelled against the king of Assyria, and served him not." — 2 Kings 18:7 (ASV)

And the Lord was with him
The Word of the Lord was for his help, as the Targum:

and he prospered whithersoever he went forth ;
that is, to war:

and he rebelled against the king of Assyria :
which is explained in the next clause:

and served him not ;
he refused to be his servant, as his father Ahaz had been, (2 Kings 16:7) , to which he was not obliged by any agreement of his; and, if it was in his power, might lawfully shake off his yoke, which is all that is meant by rebelling against him; he refused to be tributary to him.