Editor's note: William Howell is the Sydney Stein professor in American politics at the Harris School of Public Policy, University of Chicago. He is the co-author, most recently, of "While Dangers Gather: Congressional Checks on Presidential War Powers." Watch him on CNN Newsroom at 10 a.m. ET Tuesday.
(CNN) -- In 28 minutes Monday evening, President Obama made the case for intervening in Libya. He offered something of the "Goldilocks principle" for military affairs: not too much to thrust the nation into yet another war -- a word he deliberately avoided throughout his speech; not too little, which would invite humanitarian catastrophe; but just enough to set things right.
And in his explanation, Obama succeeded. And succeeded rather decisively. This, though, should come as no great surprise. The president, after all, is playing with a mighty strong hand. For starters, recent developments make for a reasonably clear case for intervention: Libyan rebels, the Arab League, and European allies all support it; Moammar Gadhafi has demonstrated his willingness to crush those who would oppose him; and with air power alone, many believe, the United States can make a great difference in the struggle.
Events in the last week also have played out to the president's favor. In reasonably short order, the United States and its allies secured the Libyan skies and struck deeply into Gadhafi's military capabilities, allowing rebels to quickly regain their advantage in the east, just as they gather themselves for what is sure to be a tougher fight in the west.
For reasons that go beyond this particular crisis, however, the stage was already set for a powerful presidential speech Monday night. Presidents -- all presidents -- enjoy extraordinary structural advantages over the conduct of foreign policy.
More than any other domestic political actor, presidents simply know more about threats to U.S. interests abroad and what is required to meet them. At least since the Korean War, which President Truman famously dubbed a mere "police action," U.S. presidents have demonstrated a clear penchant for exercising military force abroad without clear congressional support. And during this period, members of Congress have given Democratic and Republican presidents alike virtual carte blanche to wage limited, short-term military ventures as they choose.
It is of some note, then, that Obama mentioned Congress just once in his speech, and then only as a parenthetical aside. To be sure, part of the impetus for giving the speech Monday night was to defuse the criticisms coming from Capitol Hill. But Obama knows -- and he knows that members of Congress know -- that these objections will not amount to anything more than grumblings, as long as this military venture succeeds in reasonably short order.
But therein lies the rub. Should rebel advancements stall, or should fighting become protracted, or should some U.S. service member be captured and tortured on Libyan television, members of Congress, and particularly Republicans, can be counted on to lead the charge against this president.
Arguments that now merely percolate in the media will become the subject of full and vigorous hearings. And as the field of Republican presidential candidates materializes and then winnows, we can expect to hear a steady chorus of condemnation directed at Obama's policy on Libya.
The president's speech Monday night appeared strongest when it offered a defense of actions already taken. Its vision of the future, though, was notably obscure. What will the United States do if, say, rebels and government forces become locked in combat on the streets of Tripoli?
What will he do if NATO, which has just taken control over the military campaign, fails to build upon the U.S. military's progress? Short of Gadhafi's removal from power, what terms is he willing to accept for a settlement to armed conflict in the region? Not only did Obama fail to offer clear specifics to these questions, he did not even articulate principles that should guide the search for answers.
To be sure, Monday night's was just one speech. More are sure to follow. Rather than assuming a defensive posture, however, this president would do well to lean forward. It will not do to simply articulate his resolve, as he did Monday night.
He must demonstrate it. In addition to justifying past actions, he must offer a guide to future ones. He must dictate the very terms by which we, as citizens, evaluate success and failure on the battlefield, by which we judge the actions of our allies and adversaries, and by which we situate Libya within a broader discussion of our nation's interests in the region.
Obama did not do this Monday night. But if the United States' involvement in Libya amounts to anything more than a short week's worth of bombing, he had better. For then Congress is sure to awaken from its slumber, and a new front to this military campaign will open at home.
The opinions expressed in this commentary are solely those of William Howell.