Long before the 12th century, Chinese shipping had become capable of distant voyages, and Chinese merchants sailed directly to the numerous producing centres in the archipelago. The eastern Javanese ports became more prosperous than ever before. A smaller entrepôt trade also developed on the coasts of Sumatra and Borneo and in the offshore islands at the southern entrance to the Strait of Malacca. Heaps of Chinese ceramics of the 12th to 14th century provide remarkable testimony to an important trading centre at Kota Cina near modern Medan on the northeast coast of Sumatra. In consequence, the Minangkabau princes in the hinterland of central Sumatra, heirs to the pretensions of the great overlords of Shrivijaya-Palembang, were deprived of the opportunity of developing their port of Jambi as a rich and powerful trading centre. A power vacuum existed in the seas of western Indonesia, and the Javanese kings aspired to fill it.

Java had probably long been regarded as the centre of a brilliant civilization. Old Javanese became the language of the inscriptions of the island of Bali in the 11th century, and in many parts of the archipelago the contacts of trade must have spread Java's reputation as an island of scholars. A study of the grafting of Tantric ritual onto a megalithic shrine at Bongkisam in Sarawak, some time after the 9th century, provides a glimpse of cultural diffusion at work on the maritime fringes of Indonesia. Javanese cultural influence in other islands almost certainly preceded political domination.

Disunity in the Malay world and the cultural fame of Java are not sufficient to explain why the Javanese king Kertanegara (reigned 1268-92) chose to impose his authority on Malayu in southern Sumatra in 1275. It has been suggested that the king's concern was to protect Indonesia from the threat of the Mongol ruler Kublai Khan by organizing a religious alliance. But Kertanagara probably imposed his political authority as well, though his demands would have been limited to expressions of homage and tribute.

The king's activities overseas were almost certainly intended to enhance his prestige in Java itself, where he was never free from enemies. His political priorities are reflected in a Sanskrit inscription of 1289, attached to an image of the king in the guise of the wrathful Aksobhya Buddha, claiming that he had restored unity to Java; his overseas exploits are not mentioned.

The precise doctrinal contents of Kertanegara's Tantric cult are unknown. In his lifetime and after his death his supporters revered him as a Shiva-Buddha. They believed that he had tapped within himself demonic forces that enabled him to destroy the demons who sought to divide Java. The 14th-century poet Prapanca, author of the Nagarakertagama and a worshiper of Kertanagara, on one occasion refers to the king as the "Vairocana Buddha" and associates him with a ritual consort, who is, however, the consort of Aksobhya Buddha. Prapañca also admires the king's scholarly zeal and especially his assiduous performance of religious exercises for the good of mankind. The role of the royal ascetic had long been a familiar feature of Javanese kingship. The king who had been buried in the 9th-century mausoleum of Prambanan was identified with Shiva, the teacher of asceticism. Early in the 13th century King Angrok, according to a later chronicle, regarded himself as the Bhatara Guru and therefore as Shiva, the patron of ascetics. Shaivite and Mahayana priests were under royal supervision from at least as early as the 10th century, and the Tantric concept of a Shiva-Buddha, taught by Kertanagara, would not have been regarded as extraordinary. Javanese religious speculation had come to interpret Shaivism and the Mahayana as identical programs for personal salvation, with complementary gods. Union with divinity, to be achieved here and now, was the goal of all ascetics, including the king, who was regarded as the paragon of ascetic skill. Kertanagara's religious status, as well as his political problems and policies, are by no means eccentric features in early Javanese history; particular circumstances, stemming from Chinese participation in maritime trade in the archipelago, enabled him to exercise his divine power beyond Java itself. In the 14th century the homage of overseas rulers to the Javanese king was taken for granted.