Although Yan Fei could prevent the US Forces from landing on Misty Island, he was no match for them, as the Americans controlled a devastating weapon—nuclear bombs. Yan Fei had indeed seized a nuclear bomb from the Assassin Sect Headquarters, but its remote controller was broken, and he could not detonate it, rendering him incapable of threatening The United States of America.
Even if Yan Fei's nuclear bomb had been operational, he still would not have stood a chance against the US Forces, who had many more nuclear bombs than he. Even if Yan Fei could intercept the nuclear bombs launched by the US Forces at high altitude, if he missed just one, Misty Island would surely be destroyed.