the battle was fought along the irtysh river near the altai mountains. helu's forces, consisting of 100,000 cavalry, were ambushed by su as helu chased decoy tang troops that su had deployed. helu was defeated during su's surprise attack, and lost most of his soldiers. turkic tribes loyal to helu surrendered, and helu escaped to tashkent in modern uzbekistan. the retreating helu was captured the next day after residents of tashkent handed the qaghan over to the tang. on the way back to the tang capital, helu is reported to have written: