Presidential candidates make many promises they don't intend to keep. The most frequent one is that they will resist the siren song of foreign adventure and focus on pocketbook issues. The image of a president vainly trying to avoid foreign policy thickets but eventually devoting more attention to international matters than to domestic well-being is so familiar that the motto of American diplomacy may be less Teddy Roosevelt's "Speak softly and carry a big stick" than Michael Corleone's "Just when I thought I was out, they pull me back in!"
Every president in recent memory has come into office vowing to commit foreign policy only when absolutely necessary; every one has ended up making statecraft the centerpiece of his administration. Elected on that most domestic of rhetorical questions—"Are you better off now than you were four years ago?"—Ronald Reagan burnished his legend by cold-cocking communism and liberating Grenada's oppressed masses. George H.W. Bush came into office as the Education President but spent his tenure battling former clients Saddam Hussein and Manuel "Pineapple Face" Noriega. Capitalizing on Bush's homeland neglect, Bill Clinton gave us the phrase "It's the economy, stupid," then absurdly seemed to believe he would be remembered for peace breakthroughs in the Middle East, Northern Ireland, and the Balkans.
George W. Bush is a kind of summation of this trend. His campaign promised a "humble" foreign policy, and a large minority of voters deemed him fit for office despite—more likely, because of—his indifference to foreign affairs, famously signaled by an inability to name several heads of state. Yet apart from an early tax cut, he has done little beside foreign policy. He at least has a compelling reason for his foreign policy focus—though his plans, as of this writing, to prosecute a war on radical Islam by overthrowing the most secular government in the Middle East are questionable at the very least.
The temptation to lose oneself in diplomacy can't be explained by electoral calculations. Foreign policy has been conspicuously absent from voters' decision making for many years. Victory in the Persian Gulf didn't win Bush I a second term, nor did failure in Vietnam cost Richard Nixon the White House. Jimmy Carter produced fiascos in Iran and the controversial completion of the Panama Canal treaty, but he was done in by stagflation and the national malaise.
Lyndon Johnson's was the one recent presidency arguably wrecked by foreign events; but even there the actual war in Vietnam was less important than its stateside fallout, which overtook Johnson's domestic wars on poverty and abdominal scar tissue. With the raucous resistance to the war adding to the impression of a nation out of control, Nixon was elected more on the strength of his law-and-order candidacy than on his secret plan to end the conflict.
Nor can it be that foreign policy successes are easy to rack up. Even apparent triumphs in the Persian Gulf War and Afghanistan—not to mention countless doomed peace initiatives between the Israelis and the Palestinians—look less successful as time passes. As important, foreign policy usually occurs in a vacuum of public attention, its concepts and phrases—nation building, détente, peace process—as remote and ephemeral as jokes about Imelda Marcos' shoes.
So why does statecraft prove so alluring? Probably for all the reasons given above. Unlike, say, tax policy or crime prevention, foreign policy is one area where not knowing what you're doing is no barrier to entry. An amnesiac public assures that it's also largely a consequence-free game (quick: who's the current leader of Yugoslavia? Croatia? Bosnia-Herzegovina?). You can look presidential, secure in the knowledge that even your gravest errors will go unnoticed at home by all but the most committed policy wonks.
Unless, of course, your diplomatic and intelligence failures allow attacks on New York and Washington that kill more than 3,000 people. In that case, you can always blame your predeccessor or successor—whichever is more convenient.