In selecting his erstwhile opponent, Hillary Rodham Clinton, to be secretary of State, Barack Obama has gone boldly where no recent president has gone before. Unlike the lawyers, academics and generals who have dominated the office since World War II, Clinton combines substance and essential qualities for the job with celebrity and a political constituency.

For those who followed the 2008 Democratic primary campaign, this appointment might seem head-scratchingly incongruous. Wasn’t it Clinton who impugned Obama’s readiness to be commander in chief with her now-infamous ad about a 3 a.m. call to the White House? Will she now insist on being patched in to that call?

Apparently not. Both Clinton and Obama insist that, partly as a result of her tireless campaigning for him in the general election, they have made their peace and then some. It’s also true that their foreign policy differences were exaggerated in the fun-house mirror of the primary campaign. Like Obama, Clinton is willing to engage this country’s adversaries afresh, but without foreclosing military action when necessary to defend U.S. interests.

Not even her detractors would deny that Clinton is smart, well-versed in foreign and defense issues and single-minded in pursuing her goals. Her perseverance even after Obama seemed almost certain of victory augurs well for her ability to press for diplomatic breakthroughs -- in the Arab-Israeli conflict, for example -- when persistence seems futile. So too does her renown as a first lady turned senator turned diplomat.

A canny and charismatic secretary of State can be useful, however, only if allies and adversaries see her as the president’s absolute alter ego. As James A. Baker III, President George H.W. Bush’s secretary of State, observed, a foreign leader “can see daylight between a president and his secretary of State from a thousand miles away.” Such gaps are less likely when the secretary isn’t a political luminary. That may explain why the last politician of Clinton’s stature to serve as secretary of State was James Byrnes (1945-1947), a former U.S. senator and Supreme Court justice who had angled for the Democratic vice presidential nomination in 1944. (Christian Herter, a former governor of Massachusetts, served under President Eisenhower.)

Although history suggests it is difficult for rivals to collaborate on foreign policy, Clinton may be particularly well-suited for the challenge. Her high profile could help energize U.S. foreign policy while her background as a lawyer serves her in tough negotiations. She has proved to be flexible and pragmatic, as has Obama. His appointment of Clinton, whatever its complexities, shows the president-elect’s sense of confidence in her and in the administration they now will share.