DA NANG, Vietnam — In the tropical climate of central Vietnam, weeds and shrubs seem to grow everywhere — except here.

Forty years after the United States stopped spraying herbicides in the jungles of Southeast Asia in the hopes of denying cover to Vietcong fighters and North Vietnamese troops, an air base here is one of about two dozen former American sites that remain polluted with an especially toxic strain of dioxin, the chemical contaminant in Agent Orange that has been linked to cancers, birth defects and other diseases.

On Thursday, after years of rebuffing Vietnamese requests for assistance in a cleanup, the United States inaugurated its first major effort to address the environmental effects of the long war.

“This morning we celebrate a milestone in our bilateral relationship,” David B. Shear, the American ambassador to Vietnam, said at a ceremony attended by senior officers of the Vietnamese military. “We’re cleaning up this mess.”