What does Isaiah 10:8 mean?

Isaiah is quoting the Lord God. He has said He commands the armies of Assyria to come and crush His people in Israel for their rebellion against Him (Isaiah 10:6). The king of Assyria, Sargon II, does not believe for a moment that the Lord is in charge. This arrogant conqueror of nations believes himself to be the source of his own success and power (Isaiah 10:7). It would never occur to Him that the Lord God of Israel is using him as a tool to work His own will on the earth.

The Lord begins to quote how Sargon would describe his success from his point of view. He begins by pointing to the commanders of his armies, asking if they are not all kings. In other words, Sargon declares his commanders to be so powerful and feared that they have more authority and responsibility than many of the rulers of the city-states. These are the city-states they have already conquered. Those rulers called themselves "kings" until the Assyrian commanders wiped out their kingdoms and power.