Barack Obama has to show his heart is in the fight. Tonight's Afghan address must explain but also inspire. The professor-in-chief must now preach to the public about why Afghanistan is still the "good war." Why it is still worth the cost. Why the long war must be longer. Why we can win - and what exactly is to be won. The public will be listening to Obama's explanations but, perhaps more importantly, it will also be searching for the passion beneath his prose. Spock must find his inner Kirk.

"Once more unto the breach" is a difficult sell after so much deliberation. This three-month public review comes less than a half-year after his last policy review. Obama first told the nation in March of his, "comprehensive, new strategy for Afghanistan and Pakistan."

Tuesday, Obama will tell Americans of his newest strategy. The prolonged consideration, and reconsideration of those considerations, is vintage Obama. The dispassionate realism. The cold logic. The cost benefit analysis. We know the character and see it in this president.

But this is war. It's the solemn and earnest province of the presidency. Obama gets the solemn. But does he understand the earnest?

Obama's torment is transparent. He wants out of the fight, or so it seems. But he is resigned to the need to still fight. He does not want to be Lyndon Johnson but sees no way to govern as George McGovern. Obama has concluded that the costs of pulling out outweigh the costs of pushing on. Few believe he would continue this war if he had his druthers. We know he made a hard call. Now he must explain his call. Then he must convince skeptics it's the right call.

The political left is the hardest sell. Six in 10 Republicans support sending more troops into Afghanistan. Six in 10 Democrats oppose. But independents reflect Democrats views. And therefore, Obama must do more than ask an antiwar base to reconcile with this war.

Like Obama's health care legislation, the cost will come before benefits. Obama reportedly plans to increase troop levels by at least 30,000. Next year, American casualties will rise. But the benefits of this Afghan-surge may be unclear until 2011.

House Speaker Nancy Pelosi reportedly spoke last week of Democrats' "serious unrest" over Obama's decision to escalate the war. Pelosi used words like "opportunity costs." She knows that war has killed so many reformers--from Wilson to Truman to Johnson. The guns and butter scale surely worries Obama too.

We have seen this movie before: with Johnson, W. and Obama. Obama's March address spoke of how, "Al Qaeda and its allies -- the terrorists who planned and supported the 9/11 attacks -- are in Pakistan and Afghanistan." He said, "The future of Afghanistan is inextricably linked to the future of its neighbor, Pakistan." And the American future was inextricably linked to Pakistan and Afghanistan, or so he effectively argued.

He must now make much the same argument. That Americans must die not for an immediate threat but an abstraction of what could be. As Obama knows well, the "terrorists who planned and supported the 9/11 attacks" are almost entirely in Pakistan. And we are mostly in Afghanistan. Like W, Obama is a prisoner of an unstable Pakistan that he dare not further destabilize. He is facing the same whack-a-mole enemy that allows no clear sense of victory. It's a war of discordance.

The once antiwar candidate must now rally his nation in war. The man who opposed George W. Bush's "surge" in Iraq must now explain his surge. "America's commitment is not open ended," W. said in his 2007 address on Iraq. Obama must say no less. And he may go further. Obama might detail the way out of this war. But how does he say this war is worth more American lives to win, while inferring that, at some point, it could be worth losing as well?

Obama will face an ambivalent and war weary public. Eight years on, Americans have turned against this war. He must turn Americans back.

The president has other occasions to speak to the Pakistanis or to the Karzai government. He has surrogates to explain the minutia of his policy. This address regards sending more young Americans to war. He must rouse the public. The candidate who favors the subtle shades should reach for some black and white tones. It need not be good versus evil. But if Afghanistan still is the "good war," he should say so. And he should say so passionately.