Albert Barnes Commentary 2 Kings 18:20

"Thou sayest (but they are but vain words), [There is] counsel and strength for the war. Now on whom dost thou trust, that thou hast rebelled against me?" — 2 Kings 18:20 (ASV)

Hezekiah no doubt believed that in the “counsel” of Eliakim and Isaiah, and in the “strength” promised to him by Egypt, he had resources that justified him in provoking a war.

Vain words — As noted in the margin, this literally means a mere word to which the facts do not correspond.