After securing peace with Đại Việt in 1170, Cham forces under Jaya Indravarman IV invaded the Khmer Empire over land with inconclusive results.^[144] 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.^[145] 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. 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.^[146] The Khmer empire was in the verge of collapse. Jayavarman VII from the north coalesced an army to battle the invaders. He had campaigned against the Chams in his youth, in the 1140s, and participated in a campaign in Cham capital Vijaya. His army won a series of unprecedented victories over the Cham, and by 1181 after winning a decisive naval battle, Jayavarman had rescued the empire and expelled the Cham.^[147]