The expedition had a stated rationale. Kublai declared that the elderly Cham king, Indravarman V, was blameless — the resistance came from his son Harijit and a Southern Chinese ally. Whether the distinction mattered to the men loading landing crafts in late 1282 is unclear. What mattered was the destination: Vijaya, the Cham capital at the bay the Vietnamese called Thị Nại, on the coast of central Vietnam. The Yuan fleet made the passage in late 1282 and arrived at the coast near modern-day Qui Nhơn in February 1283. The Cham defenders had not been idle. They had built a fortified wooden fortress on the west shore of the bay — exactly where any seaborne invader would have to make landfall.

Sogetu's forces landed at midnight on 13 February 1283. The assault came from three directions simultaneously: 300 soldiers under Cheng Pin and Chao Ta approached from the east; 1,600 troops under Chen Chung-ta, Li Ch'uan, and Su Ch'uan moved in from the north; and the main force of 3,000, led by Sogetu himself, attacked from the south in three columns. At 05:00 AM, the Cham responded. The south gate opened, and 10,000 warriors marched to the beach — war elephants moving among the columns, drums beating, banners raised. The trebuchets, operated by Muslim engineers who had likely come to Champa through the maritime trade network, opened fire. The fighting lasted through the night and into the following day. By 01:00 AM on 14 February, Yuan forces from the north and east had breached the fort. Thousands of Cham defenders died in the assault, drowned, or were killed inside the walls.

Yuan forces reached Vijaya on 17 February 1283 and completed its capture approximately two days later. But victory at the bay was not the same as victory in Champa. Indravarman V did not surrender. Before evacuating into the western highlands, he executed two Mongol envoys — Yu Yongxian and Yilan — and burned his own warehouses to deny the invaders their supplies. He then disappeared into the mountains, where the Cham kingdom knew the terrain and the Yuan forces did not. The guerrilla campaign that followed proved far more costly to the Mongols than the beach assault had been. Yuan troops captured Vijaya, held it briefly, then withdrew outside the city to set up camps — a sign that controlling the coast was not the same as controlling the country. The occupation never consolidated.

Thị Nại Bay is now the harbor of Qui Nhơn, the capital of Bình Định Province and a modern port city on Vietnam's South Central Coast. The wooden fortress is long gone. The trebuchets and war elephants exist only in Chinese military chronicles and Vietnamese historical records. But the bay's shape hasn't changed: the same sheltered water where Mongol landing crafts grounded in the dark of 13 February 1283 is where fishing vessels and container ships now move. This shoreline saw Champa's last significant military engagement before Mongol pressure and, later, Vietnamese expansion erased the kingdom from the map entirely. The Battle of Thị Nại Bay is one of the places where that story touched ground.