KIEN VANG, Vietnam — John F. Kerry returned Sunday to the winding waterways of Vietnam’s Mekong Delta region, where he once patrolled on a naval gunboat in the search for communist insurgents as a young lieutenant in 1968 and 1969.
But nearly 50 years later, Kerry is fighting climate change and promoting sustainable aquaculture and trade in a rapidly expanding economy rather than hunting Viet Cong guerrillas, as he was at the height of the Vietnam War.
‘‘Decades ago on these very waters, I was one of many who witnessed the difficult period in our shared history,’’ Kerry told a group of young professionals gathered near a dock at the riverfront village of Kien Vang.
‘‘Today on these waters I am bearing witness to how far our two nations have come together, and we are talking about the future, and that’s the way it ought to be,’’ he said.
That future, especially for the water-dependent economy of the millions who live in the Mekong Delta, is in jeopardy, he said, pledging a $17 million contribution to a program that will help the region’s rice producers, shrimp and crab farmers, and fishermen adapt to potential changes caused by higher sea levels that bring salt water into the delicate ecosystem.
Kerry arrived in Vietnam on Saturday for his 14th trip to the country since the war’s end but his first in 13 years. He hopes to bolster the remarkable rapprochement that he had encouraged and helped engineer as a senator in the 1990s.
In the city he first knew as Saigon, the capital of the former South Vietnam, Kerry met Saturday with members of the business community and entrepreneurs to talk up the Trans-Pacific Partnership, a broad trade agreement that the United States is now negotiating with Vietnam and nine other Asian countries.
On Sunday, Kerry said he would make it a personal priority to ensure that none of the six countries that share the Mekong — China, Myanmar, Thailand, Laos, Cambodia, and Vietnam — and depend on it for the livelihoods of an estimated 60 million people exploits the river at the expense of the others.
In a pointed reference to China, which plans several Mekong Dam projects that could seriously affect downstream populations, Kerry said: ‘‘No one country has a right to deprive another country of a livelihood, an ecosystem, and its capacity for life itself that comes from that river. That river is a global asset, a treasure that belongs to the region.’’
The Mekong’s resources must ‘‘benefit people not just in one country, not just in the country where the waters come first, but in every country that touches this great river.’’
Though Kerry was keen to focus on the future, his return to the Mekong Delta, his first since 1969 despite his 13 previous postwar trips, was clearly a homecoming of sorts.
As Kerry’s boat eased off a jetty onto the Cai Nuoc River, the secretary of state told his guide: ‘‘I’ve been on this river many times.’’ Asked how he felt about returning to the scene of his wartime military service for the first time, Kerry replied: ‘‘Weird, and it’s going to get weirder.”
On this tour, Kerry was clad in drab olive cargo pants, a blue-and-white plaid long-sleeved shirt and sunglasses instead of the uniform he wore as a Navy officer. In a new role and new garb, Kerry revisited the delta’s rivers that made a vivid impression on him as a young lieutenant and eventually turned him against the war.
Standing next to the captain and surveying the brown water and muddy banks, Kerry recalled the smell of burning firewood as his boat passed through small fishing villages where the aroma hasn’t changed in 50 years.
At one point, a family in a sampan traveling in the opposite direction smiled and waved. Kerry waved back, and noticing the family had a dog on board, remarked with a smile: ‘‘I had a dog, too. Its name was VC.’’ VC was the abbreviation for the Viet Cong, forces fighting the South Vietnamese and their US allies.
Before his remarks in Kien Vang, Kerry visited a general store and bought candy for a group of children, delighting them with a few words in Vietnamese.
While the ringing of cellphones may have replaced the thunder of artillery fire, back on the boat Kerry looked out at the jungle canopy that rises just off the riverbank, swept his arm and remarked: ‘‘It hasn’t changed all that much. A lot of it is same old, same old.’’
‘‘This was what we called a ‘free-fire zone’,’’ he said. ‘‘The Viet Cong were pretty much everywhere.’’