Now therefore give pledges, I pray thee, to my master the king of Assyria, and I will give thee two thousand horses, if thou be able on thy part to set riders upon them.
Give pledges - a taunting challenge. Only give the guarantee that you can supply as many as 2,000 riders, and I will give thee 2,000 horses. But seeing that you have not even this small number (note Isaiah 2:7), how can you stand against the hosts of Assyrian cavalry? The Jews tried to supply their weakness in this 'arm' from Egypt (Isaiah 31:1).