John Kerry has visited the place in the Mekong Delta where he was ambushed during the Vietnam War.
The outgoing secretary of state and former Navy lieutenant met a 70-year-old former member of the Viet Cong, who remembers the 1969 attack.
John Kerry and his former enemy, Vo Ban Tam, warmly shook hands.
The secretary of state, who is in Vietnam as part of his last trip before leaving office, won a medal for bravery for his actions but became an anti-war campaigner after returning home.
John Kerry told Vo Ban Tam he was glad they were both alive.
Vo Ban Tam, now a shrimp farmer, said he knew a man whom John Kerry shot and killed and remembered the plan of attack when they first spotted the US patrol boat.
The Viet Cong unit had a rocket launcher and was shooting at the US fighters to try to steer them into its range.
However, John Kerry took a bold move by leaping ashore to pursue his assailants, and shot dead the rocket launcher’s operator.
John Kerry, then aged 26, was credited with saving his crew and was awarded the US military’s Silver Star for bravery.
Vo Ban Tam named the dead fighter as Ba Thanh and said he was 24 years old.
“He was a good soldier,” he told John Kerry, speaking through an interpreter.
John Kerry never knew the name or age of the man he shot.
When he unsuccessfully ran for president in 2004, John Kerry faced critics who claimed he shot a teenager.
One of John Kerry’s aides told the Washington Post that the former military man had been searching Google Maps for the site of the ambush. On January 12, he was said to have woken, jetlagged, in the middle of the night in his Hanoi hotel and called one of his old crew members to rack his brains.
John Kerry said returning to the scene was weird and a little surreal.
The secretary of state is visiting Vietnam as the first stop on his last foreign trip before stepping aside when the Trump administration takes power on January 20.
It is John Kerry’s fourth visit to Vietnam as Washington’s top diplomat.
Working under President Barack Obama, John Kerry is known for taking a specific interest in improving relations between the US and Vietnam.
He was awarded other honors for his service in Vietnam, including three Purple Hearts for being wounded in action, but he became a prominent anti-war activist after returning to the US in 1969.
According to AFP, John Kerry told reporters on January 14: “It impressed on me the notion that you really need to analyze and understand what lies underneath the slogans.”