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Back on the boat, Mr. Kerry looked out at the jungle canopy that rises just off the riverbank.

“It hasn’t changed all that much. A lot of it is same old, same old,” he said.

“This was what we called a ‘free-fire zone’,” he said. “The Viet Cong were pretty much everywhere.”

Mr. 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.

“When I came home after two tours of duty, I decided that the same sense of service demanded something more of me,” he wrote in his 2003 book, “A Call to Service,” as he was unsuccessfully campaigning for the presidency in the 2004 election.

“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.”

On his first trip to Vietnam as secretary of state, he was determined to bolster the remarkable rapprochement that he encouraged and helped engineer as a senator in the 1990s.

In Ho Chi Minh City, which he first knew as Saigon, the capital of the former South Vietnam, Mr. Kerry on Saturday met with members of the business community and entrepreneurs to talk up a trade agreement the U.S. is negotiating with Vietnam and nine other Asian countries.

To take full advantage of the deal’s economic opportunities, Mr. Kerry said that Vietnam, which has been widely criticized for its human rights record, must embrace changes that include a commitment to a more open society, the free exchange of ideas, and education.

He spoke after attending Mass at Notre Dame Cathedral, built in the 1880s and 1890s under French colonial rule, in a bid to show support for the tenuous freedom of worship in Vietnam. Vietnamese authorities have been criticized for harassing, prosecuting and jailing Catholic clergy.

By the end of Sunday, he was in Hanoi, where he planned talks with government officials Monday that are expected to focus on maritime security and territorial disputes in the South China Sea.

He also was expected to make the case that respect for human rights, particularly freedom of speech and religion, is essential to improved relations with the United States, as well as raise the issue of political prisoners.