She could be like Philander Knox, who served as secretary of state from 1909-13 and returned to the Senate in 1917. Or she could be more like Ed Muskie—the last senator to serve as secretary of state. Muskie ended his political career in 1980, when Jimmy Carter's loss to Ronald Reagan forced him out of the position just seven months after he had accepted Carter's offer.
But the closest parallel to someone like Hillary Clinton becoming secretary of state is the case of James Byrnes. Although he wasn't a senator when he was picked by Harry Truman—he had served in the Senate earlier in his career—Byrnes had been a close adviser to Truman's predecessor, Franklin D. Roosevelt (not unlike Clinton to President Clinton), and he was also Truman's rival, not for the presidency (as was the case for Clinton and Obama) but for the VP slot.
As President Barack Obama's foreign-policy agent, Hillary Clinton will have the problems all these secretaries faced—and then some. Only a true believer can envision Obama and Clinton making a good team. You have to believe in Obama's ability to control Clinton's independence, believe in Clinton's capacity to execute someone else's policies, believe in the ability of these two rivals to suddenly become close, believe that knowledge and experience are not crucial for the job, believe that the complicated Clinton family drama will not be a problem, believe that policy differences can always be bridged, and believe that it's possible to be both an ambitious politician and an honest-to-God civil servant.
Most of all, you have to believe that "change" can come not to nations alone—but also to people, even to politicians.
As Truman's secretary of state, Byrnes was not a great success. "Maddeningly independent," as David McCullough described him in Truman, Byrnes sometimes forgot that Truman was president. He eventually resigned, clearing the way for the more successful, more submissive George Marshall. Will Clinton always remember that Obama is president and that she isn't? Can she also forget, for a while at least, that she wants to return to the White House someday?
It has been a very long time since an active politician was able to overcome his ambition and become a truly effective secretary of state. The last to make the switch successfully was Cordell Hull, back in 1933, but Hull never returned to politics. It's doubtful whether Clinton plans to follow that path, just as it's hard to imagine her serving as secretary for 11 years, the way Hull did. In four years, or even eight, Clinton will still have plenty of time and energy. She'll want to do something with it.
Clinton might have more foreign-policy experience than Obama, but her record is still skimpy. Her most impressive achievement in this field may be the speech she gave in Beijing at the United Nations' fourth world conference on women. That was in 1995. In these days of celebrity culture, it's easy to forget that the most successful secretaries of the era were either knowledgeable experts (Henry Kissinger), experienced practitioners (James Baker), or both (Marshall, Dean Acheson). And they all had one advantage that Clinton will not enjoy: They served under presidents (Harry Truman, Richard Nixon, and especially George H.W. Bush) whose keen interest in foreign affairs long preceded their decisions to run for president.