The arrival of the Carl Vinson strike group’s 5,500 sailors represents the first time such a large contingent of American military personnel has landed on Vietnamese soil since the last of the United States troops withdrew in 1975.

During the four-day port call, the aircraft carrier’s personnel will visit an orphanage and a center for victims of Agent Orange, the defoliant used by the United States military that is blamed, through a toxic contaminant, for poisoning generations of Vietnamese.

Carrier sailors will also play basketball and soccer with Vietnamese counterparts.

For the last month, the Carl Vinson has been deployed in the South China Sea, one of the world’s busiest shipping lanes. Six governments have competing claims over various features in the South China Sea. In recent years, Vietnam, in particular, has watched warily as China, through extensive reclamation, has transformed bits of rock and reef it controls into sprawling artificial islands that now double as military bases.

In 2017 alone, China built permanent facilities on reclaimed land that “account for about 72 acres, or 290,000 square meters, of new real estate,” according to the Asia Maritime Transparency Initiative run by the Center for Strategic and International Studies in Washington.

“Hanoi’s agreement to the aircraft carrier visit demonstrates Vietnam’s anxiety about what China will do next in the South China Sea,” said Murray Hiebert, senior associate of the Southeast Asia Program at the Center for Strategic and International Studies. “The U.S. is virtually the last man standing to which Hanoi can look for support in the South China Sea dispute.”