Ewald-Heinrich von Kleist, the last known surviving conspirator of Operation Valkyrie, the 1944 assassination plot that failed to kill Adolf Hitler but became a celebrated episode of German resistance during World War II, died March 8 at his home in Munich. He was 90.

His wife, Gundula von Kleist, confirmed his death to the Associated Press. The cause was not reported.

To his admirers in Germany and around the world, Mr. von Kleist represented a flash of heroism, however futile, in one of the bleakest moments of 20th-century history.

He was born into an aristocratic Prussian family that had produced a long line of cultural, diplomatic and military leaders. Mr. von Kleist followed in their stead, becoming a Wehrmacht lieutenant while still in his early 20s.

But like his father, a well-placed German professional, Mr. von Kleist became deeply disturbed by Hitler’s leadership and the brutality of the Nazi apparatus. The father and son joined a circle of aristocratic and military leaders who mounted an opposition against the Fuehrer.

Ewald-Heinrich von Kleist, here in a 1978 photo, volunteered for a suicide plot to assassinate Adolf Hitler. (Hartmut Reeh/AP)

Operation Valkyrie — the attempt on July 20, 1944, to assassinate Hitler by detonating a bomb-laden briefcase at his eastern headquarters — became their most famous exploit and was popularized in a 2008 film starring Tom Cruise as Claus von Stauffenberg, the plan’s ringleader.

Other attempts preceded it, including one in early 1944, after Mr. von Kleist had been wounded on the Eastern Front. He was recuperating when he received a telegram recalling him to his unit. He assumed he would be required to resume his duties, but when he arrived, he received an unexpected request: to carry out a suicide mission to assassinate the Fuehrer.

The telegram had come from von Stauffenberg, an army colonel. Mr. von Kleist asked for 24 hours to think it over. In an interview with the Vancouver Sun in 2000, he recalled his conversation with his father:

“My father said to me, ‘You have to do it. If you fail at such a moment, you’ll never be content again in this life.’ I decided to do it. I don’t think it was a question of courage. There is such a thing as conscience.”

According to the plan, Mr. von Kleist would be the officer assigned to model a new uniform at an inspection by Hitler. His uniform, however, would be strapped with explosives. Before the plan could be carried out, Hitler’s plane was diverted, and the opportunity passed.

Months later, Mr. von Kleist very nearly became the central figure in Operation Valkyrie. Von Stauffenberg had initially asked Mr. von Kleist to carry the briefcase into the Wolf’s Lair, as Hitler’s eastern headquarters was known.

In a last-minute change, perhaps because Mr. von Kleist was too low-ranking to plausibly present himself at such a location, von Stauffenberg decided to perform the operation himself.

He carried the briefcase into the meeting room and strategically placed it near Hitler’s chair. In an unwitting move that has sparked countless what-ifs, someone shifted the briefcase just enough so that when the bomb detonated, Hitler was spared the brunt of the explosion.

The bomb killed several others in the room, but — unbeknownst to von Stauffenberg as he raced back to Berlin — Hitler emerged alive. Von Stauffenberg rendezvoused with other conspirators including Mr. von Kleist, who had moved forward with the putsch that was intended to follow Hitler’s death.

In the interview with the Vancouver paper, Mr. von Kleist recalled his response when he learned of Hitler’s survival. “I went to Stauffenberg and said, ‘We’re going to have some problems now.’ ”

Hitler and his loyalists retaliated with fierce reprisals, killing von Stauffenberg, other conspirators and scores of perceived enemies. Among those executed was Mr. von Kleist’s father.

Mr. von Kleist was interrogated by the Gestapo for weeks after the attack.

“I thought of the lines from the Divine Comedy: ‘Abandon hope, all ye who enter here,’ ” Mr. von Kleist told an interviewer, referring to the noted passage from Dante’s “Inferno.”

He survived the interrogation, he once told the London Evening Standard, by playing “the role of being young and stupid and un­political.” After the interrogation, he was placed in the Ravensbruck concentration camp but — inexplicably — was released several months later to return to the front.

Ewald-Heinrich von Kleist was born July 10, 1922, on his family’s estate in Pomerania, a region of northeastern Germany that is now part of Poland. He studied architecture before joining the German army.

He said that he became disaffected with the Nazi Party after the “Night of the Long Knives” in 1934, when the Nazis murdered political enemies. His father nearly lost his life.

“It was absolutely clear that the state had become a murderer,” Mr. von Kleist told the London Evening Standard. “Later on, many people said they knew little or nothing of what was happening, which was mostly true. But this was in the newspapers.”

After the war, Mr. von Kleist worked in publishing and founded the Munich Security Conference, an annual meeting attended by top political leaders from around the world. For shepherding the conference, he received the U.S. Defense Department’s Medal for Distinguished Public Service, the highest honorary award the department bestows on private citizens.

Survivors include his wife, the former Gundula Freude.

Years later, with the clarity or in some cases arrogance of hindsight, critics have said that the Operation Valkyrie plot was too little, too late. “We had to try something,” Mr. von Kleist told the Canadian National Post in 2004. “The things being done by those criminals in Germany’s name were simply appalling.”