American Vietnam veterans have helped their former enemies find the remains of 81 soldiers in a mass grave, in the latest sign that the U.S.-Vietnamese relationship is as strong as it’s been since the end of the war.

The Vietnamese military said the U.S. vets helped them locate the grave of soldiers who died over 40 years ago, during what was believed to be a Vietnamese attack on an American base in the Central Highlands. U.S. veterans who served at the base gave the Vietnamese government a map of where they buried the soldiers, and the remains of four of them have been returned to their families. The remaining 77 unidentified remains have been buried in a local military cemetery.

The discovery of the graves is the latest success for a joint U.S.-Vietnamese program that was started in the 1990s to find soldiers still listed as missing in action (MIA).

The ongoing missions have already recovered and identified the remains of over 700 Americans killed in Vietnam, Cambodia and Laos. Nearly 1,800 US military personnel still remain missing. And dozens of remains of MIA Vietnamese soldiers have also been identified - with the help of U.S. veterans’ groups.

This level of cooperation didn’t exist as late as the 1990’s, when relations between the former warring nations were much more tense. At the time, it wasn’t unusual for Americans to avoid identifying themselves, particularly in areas of former conflict around Danang and Hue, in the center of the country.

Then came President Bill Clinton’s 1994 decision to lift a trade embargo against Vietnam, a move he said was intended to secure cooperation from the government there in getting information about the MIAs. That led to the 1995 normalization of diplomatic relations, which in turn expanded trade and other contact between the two nations.

The work has continued ever since on trying to find the remains of MIAs, an effort often led by U.S. veterans’ groups.The search for remains often involves interviewing witnesses – former U.S. and Vietnamese officials – about incidents like the downing of a U.S. military plane. Encouraging information is then passed along to a joint team that sets about the painstaking work of digging in jungle terrain to try to find remnants of the crash, and pilot remains.

In the first years after the war, the Vietnamese government could spare few resources to look for the remains of their own soldiers as they rebuilt their war-ravaged country. That has changed in recent years, largely because their officials have gained experience of looking for remains by working with American teams.