KIEN VANG, Vietnam - Along the winding muddy waters of the Mekong Delta where he once patrolled for communist insurgents on a naval gunboat, Secretary of State John Kerry turned his sights Sunday on a new enemy: climate change.

In this remote part of southern Vietnam, rising sea waters, erosion, and the impact of upstream dam development on the Mekong River are proving a more serious threat than the Viet Cong guerrillas that Kerry battled as a young lieutenant in 1968 and 1969.

"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 fisherman adapt to potential changes caused by higher sea levels that bring saltwater into the delicate ecosystem.

Kerry also 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 13 previous postwar trips to Vietnam, was clearly a homecoming of sorts.

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.

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."

Kerry first set foot in Vietnam 44 years ago after volunteering for service because, as he has said, "It was the right thing to do." He was decorated with three Purple Hearts, a Silver Star, and a Bronze Star for fighting in a conflict that he came to despise and call a "colossal mistake," one that profoundly influenced his political career and strategic view.

"The lesson I learned from Vietnam is that you quickly get into trouble if you let foreign policy or national security policy get too far adrift from our values as a country and as a people."

He arrived back on Saturday for his 14th trip to the country since the war's end but his first in 13 years.