As a young naval officer in Vietnam, John Kerry commanded a Swift boat up the dangerous rivers of the Mekong Delta. But when he returned there last month as secretary of state for the first time since 1969, he spoke not of past firefights but of climate change.

"Decades ago, on these very waters, I was one of many who witnessed the difficult period in our shared history," Mr. Kerry told students gathered on the banks of the Cai Nuoc River.