“He put the question to me explicitly — ‘The time is coming when something must be done,’ ” Freya von Moltke said. “ ‘I would like to have a hand in it, but I can only do so if you join in too,’ and I said, ‘Yes, it’s worth it.’ ”

So, with a wife’s assent, began a famous challenge to Hitler. At the height of the Nazi victories, Count Helmuth James von Moltke invited about two dozen foes of Nazism, many of them aristocrats like himself, to imagine a new, better postwar Germany.

For him, his wife’s participation was essential, as she remembered the conversation in “Courageous Hearts: Women and the Anti-Hitler Plot of 1944,” a 1997 book by Dorothee von Meding.

The dissidents met at the count’s ancestral estate, Kreisau, which Bismarck had given his legendary great-great-uncle, Field Marshal Helmuth von Moltke the Elder, for his victories over Austria and France.

It was a perilous act of resistance. As many as half of the dissidents were later executed, some for actively plotting to kill Hitler, others for thinking the unthinkable: they had marshaled logical, moral and religious arguments to question the legitimacy of the Third Reich. Their high-minded planning for a future without Nazis angered a regime that expected to endure 1,000 years.

Mrs. Moltke, who disdained the title of countess, was the last living active participant in the group. She died of a viral infection on Jan. 1 at her home in Norwich, Vt., her son Helmuth said. She was 98.

In his book “The Rise and Fall of the Third Reich” (1960), William L. Shirer said the Kreisau circle had provided “the intellectual, spiritual, ethical, philosophical and, to some extent, political ideas of the resistance to Hitler.”

They initially rejected violence, if only for fear of making Hitler a martyr. But as the killing went on, support for assassinating him grew. Indeed, military conspirators were pushing ahead. At 12:40 p.m. on July 20, 1944, a bomb they had planted in a suitcase beneath a table at which Hitler was sitting at the Wolf’s Lair field headquarters exploded. Hitler suffered only minor wounds.

Mrs. Moltke said she believed that her husband would have backed that assassination attempt — had he not already been in jail for warning a friend, Dr. Otto Kiep, who was plotting violence against Hitler, that Dr. Kiep risked imminent arrest. Count Moltke was never released. He was hanged, most likely by piano wire, in January 1945 after Gestapo agents had linked the assassination attempt at Wolf’s Lair to the Kreisau circle.

In fact, there is strong evidence that Count Moltke was in contact with the July 20 conspirators. Andreas Hermes, one of the few ringleaders who were not executed, told The New York Times in July 1945 that he “vividly” recalled Count Moltke’s participation.

Women who joined their husbands to oppose Hitler treaded the same dangerous ground as the men. Mrs. Moltke could have faced the death penalty simply for serving food and drinks to the conspirators. Her husband relied on her first impressions of people to make life-and-death judgments. She contributed ideas, particularly on legal issues, and her expertise.

In an enduring contribution, she gathered up Kreisau circle documents and letters from her husband and hid them in the estate’s beehives. In 1990 she published them as “Letters to Freya.” The papers have proved valuable to scholars for their gripping portrayal of heroic, almost certainly futile resistance, as well as for their glimpses at daily life in the Third Reich. In her later years, when the German government and others came to recognize her contributions, Mrs. Moltke expressed gratitude on behalf of other resistance widows as well. “We were all wives of our husbands,” she said.

Mrs. Moltke’s son said in an interview that the only surviving wife of a Kreisau circle member is Clarita von Trott, whose husband, Adam, played a central part in the July 20 plot and was executed in 1944.

Freya Deichmann, whose father was a banker, was born in Cologne on March 29, 1911. She attended idealistic work camps that brought young people of all classes together to share ideas and dreams. At 18, while on a vacation to Austria’s lake district, she met Count Moltke.

They married two years later, in 1931. He studied in Germany and Britain to become an international lawyer. In 1939, he was drafted to work in military intelligence. His boss, Adm. Wilhelm Canaris, a covert foe of Hitler, encouraged him to use his legal and political expertise to save Jews and curb German atrocities. On trips abroad, he met with Allied officials to discuss a possible coup.

Mrs. Moltke was haunted by the first time she saw Hitler in 1931 or 1932. She had noticed a man in a dark movie theater, she told Ms. von Meding in “Courageous Hearts.” “I thought to myself, what terrifying eyes,” she said.

When the house lights went on, she saw who he was.

Mrs. Moltke earned a doctorate in law from Humboldt University in Berlin in 1935. She then took over management of Kreisau, then in eastern Germany and now part of Poland. The Kreisau circle began informally among friends, then became more serious as members assembled in small groups in Berlin to discuss specific subjects, like a new constitution. Larger meetings at Kreisau occurred in the spring and fall of 1942 and the spring of 1943.

The circle and the Moltke family benefited from the immense prestige of Count Moltke’s military ancestor. Another protection was the fervent pro-Nazi views of the manager of the estate; his local stature helped contain public denunciations of a family that refused to say “Heil Hitler.”

Mrs. Moltke’s letters to Count Moltke in prison concerned farm matters, like the wisdom of keeping a pair of ducks. She told her husband that the Gestapo officer who read all his letters had spoken nicely to her on a visit.

“They’re not really so bad,” she ventured.

“Except when they tear out your fingernails,” Count Moltke answered (though he was not tortured in that way, he said).

After the war, Mrs. Moltke moved to South Africa and did social work, but she grew to hate the country’s official system of racial segregation and eventually returned to Germany.

She moved to Vermont in 1960 to join the philosopher Eugen Rosenstock-Huessy, whose wife had died. They had met in the youth camp movement and, after reuniting, remained companions until his death in 1973.

In 1998, she helped turn Kreisau — Krzyzowa in Polish — into a center to promote understanding between Germany and Poland. In 2004, a foundation named after her was set up to support it.

Besides her son Helmuth, Mrs. Moltke is survived by six grandchildren and 10 great-grandchildren. Another son, Konrad, died in 2005.

The letters Mrs. Moltke hid in the beehives remain poignant. In the last one he wrote before his execution, Count Moltke said he would “gladly accompany” his wife “a bit further on this earth.”

“But then I would need a new task from God,” he continued. “The task for which God made me is done.”