Richard Nixon
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Richard Nixon does not appear to have attended the funerals of any soldiers killed in Vietnam. He did award posthumous medals of honor to the families of several soldiers on 22 April 1971 and on several other occasions. On Veterans day in 1971 he visited the Tomb of the Unknown Soldier in Arlington Cemetery. In 1973 he met with the family of Colonel William Nolde after the colonel was buried in Arlington Cemetery. Colonel Nolde was killed on January 27th, the night before the cease-fire went into effect.
Jimmy Carter
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According to the New York Times, Jimmy Carter attended a memorial service for the soldiers killed in the failed rescue of America hostages in Iran in 1980.
Ronald Reagan
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Ronald Reagan attended memorial services on several occasions for American soldiers. In 1983 he attended a service at Camp Lejeune, North Carolina, in connection with the bombing of the Marine barracks in Beirut, which cost the lives of 241 people. In 1987 he attended a service at Mayport Naval Station in Florida for the sailors killed on the USS Stark.
George H.W. Bush
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President George Herbert Walker Bush does not appear to have attended any funerals for American soldiers. (The NYT, citing Marlin Fitzwater as a source, indicated that the president did attend several such funerals. But no details were provided.)
Bill Clinton
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Bill Clinton attended a service in October 2000 in memory of the 17 sailors killed in the attack on the USS Cole.
After the terrorist bombing the Murrah building in downtown Oklahoma City he publicly grieved with the families of the victims at an event that was regarded at the time as a turning point in his presidency.
Like many other presidents he visited the Tomb of the Unknown Soldier.
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He can figure out how to have photo ops with the soldiers on other occasions, this should be a priority. Of course, there is also the possibility that some families don't want him there.