WASHINGTON -- The experiences that Ashton Carter would bring to the job of U.S. secretary of defense are vastly different from Chuck Hagel's in several important ways, starting with the fact that Carter is an academic and a policy wonk.
How Carter would pursue President Barack Obama's defense agenda -- or use his influence to bend it in new directions -- is less obvious.
Obama was to announce Friday that he will nominate Carter as Hagel's successor. Carter would, if confirmed by the Senate, be the fourth Pentagon chief of Obama's presidency, following Robert Gates, Leon Panetta and Hagel.
Carter has left an extensive paper trail from his many years in Washington, including three stints at the Pentagon, a trail that suggests a sometimes hard-nosed view of policy puzzles like North Korea and Russia.
In 1999 Carter and former Defense Secretary William Perry co-wrote a book, "Preventive Defense," spelling out dangers that, if mismanaged, could grow into "true A-list-scale threats" to the U.S.
First on their list: the risk that "Russia might descend into chaos, isolation and aggression." They called this the risk of a "Weimar Russia," harking back to the failures of the international system in dealing with xenophobic Germany after World War I. They wrote of their fear that Russia "could fall prey to its worst tendencies."
Russia today is among the biggest policy challenges facing the Obama administration, with its annexation of Crimea in March and subsequent military moves inside eastern Ukraine.
North Korea is on the worry list, too, with its defiance of the West and its ambition to develop a missile capable of hitting the U.S. with a nuclear warhead.
In June 2006, amid worldwide concern that North Korea was preparing to test such a long-range missile, Carter and Perry co-authored an opinion article in The Washington Post that took a hard line against North Korea.
"Should the United States allow a country openly hostile to it and armed with nuclear weapons to perfect an intercontinental ballistic missile capable of delivering nuclear weapons to U.S. soil?" they wrote. "We believe not."
If North Korea were to persist in its launch preparations the United States should "immediately make clear its intention to strike and destroy" the missile before it could be launched, they wrote. That could be done, Carter and Perry said, by hitting it with a submarine-launched cruise missile armed with a high-explosive warhead.
"The carefully engineered test bed for North Korea's nascent nuclear missile force would be destroyed, and its attempt to retrogress to Cold War threats thwarted," they added.
Unlike Hagel and every other man who has led the Pentagon over the past 30-plus years, Carter has served neither in the military nor in Congress. Hagel did both. And although he moved in national security circles for years, including his time as head of the Atlantic Council, Hagel was never seen as a master of policy.
The last defense secretary with neither experience in uniform nor in national politics was Harold Brown, who led the Pentagon from 1977 to 1981 under President Jimmy Carter. The Pentagon historian's office calls Brown the first scientist to serve as secretary of defense. He, like Carter, was a physicist by training.
A relative unknown figure nationally, Carter has served in the Pentagon under two Democratic presidents -- Bill Clinton and Obama.
Carter, 60, would be among the younger defense secretaries of recent years and the first to come of age after the Vietnam War, a conflict that was central to Hagel's frame of reference. Hagel served in combat in Vietnam as an enlisted soldier and was wounded twice.
Although Carter never served in uniform he spent a lot of time around deployed troops in recent years, especially during his stint as deputy defense secretary from 2011 to 2013. Carter has taken a highly unusual path to the brink of becoming head of the world's largest military organization. At Yale University he got undergraduate degrees in two subjects: physics and medieval history.
"There was no relationship between them in my mind except that both fascinated me," he wrote in a 2007 autobiographical sketch published by Harvard University when he was on the faculty of the Kennedy School of Government.
"I like dusty archives, learning to decipher manuscripts in medieval script, and learning all the languages necessary to read the primary and secondary historical literature, especially Latin," he wrote.
He later got his doctorate in theoretical physics at Oxford University as a Rhodes scholar.