After securing peace with Đại Việt in 1170, Cham forces under Jaya Indravarman IV invaded the Khmer Empire over land with inconclusive results.^[38] That year, a Chinese official from Hainan had witnessed elephant duel battles between Cham and Khmer armies, henceforth convincing the Cham king to offer war horse purchases from China, but the offer was rejected by the Song court multiple times. In 1177, however, his troops launched a surprise attack against the Khmer capital of Yasodharapura from warships ploted up the Mekong River to the great lake Tonlé Sap and killed the Khmer king Tribhuvanadityavarman.^[39] Multiple-bow siege crossbows were introduced to Champa from Song dynasty in 1171, and later were mounted on the backs of Cham and Vietnamese war elephants.^[40] They were deployed by the Cham during the siege of Angkor, which was lightly defended by wooden palisades, leading to the Cham occupation of Cambodia for the next four years.^[40]