He died Friday at his home in Munich, his wife, Gundula von Kleist, told the Associated Press. The cause was not announced.
As a 21-year-old lieutenant, von Kleist accepted Colonel Claus von Stauffenberg's personal request to make an attempt on Hitler's life. The young officer planned to conceal two hand grenades under his coat and to detonate them in the Fuehrer's presence at a demonstration of military uniforms in February 1944. The suicide mission never took place because Hitler was unable to attend on several occasions. It was one of at least 40 known attempts to kill Hitler.
Von Kleist then joined his father — who had endorsed his son's assassination plan — in the Operation Valkyrie plot led by von Stauffenberg and Maj. Gen. Henning von Tresckow. On July 20, 1944, the group of senior officers and supporters, disillusioned by the mass killings of Jews and others in Eastern Europe, staged a synchronized coup with the backing of a reserve army. Von Stauffenberg failed to kill Hitler with a bomb that exploded at the Wolf's Lair headquarters in East Prussia, and the conspirators were quickly rounded up by the Gestapo.
About 600 people were arrested and about 150 were executed for their roles in the plot, said Johannes Tuchel, head of the German Resistance Memorial Center in Berlin. Von Stauffenberg and von Kleist's father were among those killed. The plot was the subject of a 2008 film, "Valkyrie," starring Tom Cruise in the von Stauffenberg role.
At the reserve army headquarters in Berlin, where von Stauffenberg was chief of staff to General Friedrich Fromm, von Kleist played a supporting role during the plot. Von Kleist's involvement was investigated until December 1944, when he was sent to a concentration camp and then to the eastern front.
"I had something that is no longer common today but played a big role back then: a strong attachment to my people and my country," von Kleist told the weekly newspaper Junge Freiheit in 2001. "I found it appalling that such crimes were committed in the name of Germany."
Von Kleist was born July 10, 1922, on his family estate in Schmenzin, a town in the Pomerania region that is now part of Poland. His ancestors had served in Prussian military and administrative positions for centuries.
His father resigned from public duties when Hitler took power in 1933. In August 1938, the elder von Kleist met with Winston Churchill, a Conservative Party parliamentarian at the time, in London to gauge his support for an army revolt against the Nazi regime and to steer the British away from a policy of appeasement.
Ewald-Heinrich von Kleist was sent to high school at the Gymnasium Birklehof, near Freiburg in southwest Germany, where he was exposed to liberal ideas with French and American influences. After his schooling, he returned to Pomerania to pursue an apprenticeship in agriculture. He joined the German army at age 18 and volunteered for Infantry Regiment 9, which was known to have many officers and troops with anti-Nazi sympathies.
After the war, he started his own publishing business and founded the Wehrkundetagung, later known as the Munich Security Conference, in 1962. He stepped down in 1998 as chairman of the conference, a foreign-policy forum for government ministers, academics, high-ranking army officers and media.
Von Kleist and his wife had a son, Christian, and a daughter, Vera.
"The attempt to save millions of lives was worth it," von Kleist said in a remembrance speech for the 100th anniversary of von Stauffenberg's birth in November 2007.