 In 630, Emperor Taizong defeated the Eastern Turkic Khaganate, an ally of Goguryeo, bringing much of Central Asia under Tang control; he then conquered the Tarim Basin, and defeated the Tibetan Empire in 640.In 643, Queen Seondeok of Silla requested military aid against the Goguryeo–Baekje alliance.Tang forged an alliance with Silla, and began preparations for a major campaign against Goguryeo in 644.