WASHINGTON — 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. He drew a connection from the Mekong Delta’s troubled past to its imperiled future. “This is one of the two or three most potentially impacted areas in the world with respect to the effects of climate change,” he said.

In his first year as secretary of state, Mr. Kerry joined with the Russians to push Syria to turn over its chemical weapons, persuaded the Israelis and Palestinians to resume direct peace talks, and played the closing role in the interim nuclear agreement with Iran. But while the public’s attention has been on his diplomacy in the Middle East, behind the scenes at the State Department Mr. Kerry has initiated a systematic, top-down push to create an agencywide focus on global warming.

His goal is to become the lead broker of a global climate treaty in 2015 that will commit the United States and other nations to historic reductions in fossil fuel pollution.