If Ahaz is smart enough to believe in Isaiah, he would not need to trust in the power of another nation. For centuries the Lord had protected the Jews from the military conquests of their neighbors. This was done in spite of huge numerical mismatches. The Lord had demonstrated in the past that He was able to fight their battles for them. Such was His promise again. The tone of the next verses demonstrates that Ahaz lacked faith in this promise.
At any rate, Ahaz was able to defend his people without the help of Assyria. This is recorded in 2 Kings 16:5-6, ’Rezin…and Pekah…came up to Jerusalem to war; and they besieged Ahaz, but could not overcome him.’ In the siege, Rezin is only able to take one city, Elath. Josephus explained that after Ahaz had successfully repelled the Rezin/Pekah alliance, he took all the credit himself, forgetting the help he had received from the Lord. He then considered himself so great that he went to Damascus to attack Rezin and Pekah. This time he did not have the Lord’s help and was beaten severely. According to 2 Chron 28:6-7, 120,000 men were killed and another 200,000 were taken captive. These prisoners were later released.
This great devastation happened because of the wickedness of Ahaz. Had he trusted in the Lord and recognized his hand in his military victories, the Lord would not have let this military disaster come upon his people. It was this disaster which prompted Ahaz to get revenge. It was at this point that Ahaz enlisted the help of the Assyrians. The Assyrians then attacked the Syrians and killed Rezin. See Josephus, Antiquities of the Jews, Book IX, chapter XII, 2 Kings 16:7-9, and 2 Chron. 28.