The Assyrian envoy assumes reliance is on something tangible, something he can see and understand. He's completely missed the point.
The Assyrian king, the 'great king,' prided himself on his vast empire and military dominance. He expected Hezekiah to be basing his resistance on worldly power – perhaps Egypt's promised, but unreliable, aid. The Rabshakeh is essentially saying, 'Look at my king's power! Look at the nations we've crushed! What do you have to stand against this?'
This reveals a fundamental misunderstanding of spiritual warfare. Hezekiah's trust wasn't in chariots and horses, but in the Lord God Almighty. The Rabshakeh's question, intended to expose weakness, actually highlights the arrogance of relying solely on human strength and visible power. True security is found not in what we can muster, but in who we know.