Barack Obama's selection of former Nebraska Republican Sen. Chuck Hagel as his choice to be the nation's next defense secretary has received a tidal wave of criticism. But Hagel may be in for a bruising confirmation battle, the discussion of the pick generally misses an odd historical fact that reveals one of the ways modern Democrats blunt charges that they are soft on military issues — they pick a Republican to head the Defense Department.

Ever since 1940, Democrats have regularly chosen Republicans to serve as the top cabinet officers for defense and war issues — in fact, for more than half of the 36 years of Democratic administrations since 1940, Republicans have led the department. 

This is no quirk. While the selection of a token bipartisan member has become de rigueur over the last few decades, these choices usually end up heading second-tier departments like Transportation. Consider the four marquee cabinet positions — state, defense, treasury, and attorney general. Other than defense, there are almost no cross-party picks. In the last century, there have been no secretaries of state or attorney generals who crossed party lines. There have been three treasury secretaries who crossed party lines (and current Treasury Secretary Tim Geithner is an independent), but even those deserve asterisks. For example, Richard Nixon's second treasury secretary, Democrat John Connally, served only for a short time, and became a Republican mere months after leaving treasury.

Defense is different. Not on the Republican side of the aisle — no Democrat has been selected to lead the defense department (or its predecessor, the war department) by a Republican president in the last 100 years. But since FDR, seven Republicans have been chosen by Democrats to head defense. 

This pattern started in 1940. Franklin Roosevelt chose two old Republican hands, Henry Stinson (a former secretary of state) and Frank Knox to serve as the heads of the war and navy departments, in large part to create a bipartisan team in the face of World War II. 

In 1945, Truman replaced Stinson with Robert Patterson, also a Republican. Truman's next two choices, James Forrestal, the first person to serve in the newly created secretary of defense position, and Louis Johnson, were Democrats. But Truman soon went back to Republicans, selecting Robert Lovett.

While people may have expected the end of WWII and the Korean War to the end this unusual bipartisanship, it was actually just getting started. John Kennedy selected Republican Robert McNamara as his defense chief, a position that McNamara held until the very end of the Lyndon Johnson administration. While Jimmy Carter didn't take a Republican, Bill Clinton looked to former Republican Sen. William Cohen as his third defense secretary. Barack Obama continued the tenure of Republican Robert Gates for the first two and a half years of his first term, and now looks to another Republican for the start of his second. 

What explains this phenomenon? Some of it may be simply that the president "clicked" with that particular candidate. But there may be another reason. Ever since Vietnam — really, ever since the Cold War started — Democrats have been particularly vulnerable on military matters. Republicans have scored significant political victories with charges that Democrats are soft on communism or terrorism and want to cut defense budgets. By choosing Republicans to lead the department, Democratic presidents are inoculating themselves against the "weak on defense" charge.

Mind you, this inoculation is of limited value — no Republican is going to stop attacking a Democratic president on political or policy grounds simply because he took a fellow Republican as his defense secretary. This is especially true in the case of Chuck Hagel, who appears not to have many friends on either side of the aisle. Additionally, both Clinton and Obama chose Republicans at the start of their second term, when the overall electoral ramifications were clearly diminished. But this very obvious and continuing pattern shows that there must be a perceived value in Democrats moving across the aisle for their defense picks.

Joshua Spivak is a senior fellow at the Hugh L. Carey Institute for Government Reform at Wagner College. He writes the Recall Elections Blog.