President Obama has finally made a decision on Afghanistan, which he will communicate to an anxious nation tomorrow night. The White House announcement no doubt has already fueled many kitchen-table conversations during this Thanksgiving weekend, maybe even heated debates. For the sake of our national security, let’s hope most families have concluded that abandoning Afghanistan without achieving our aims is not an option.

Some will still waver of course, which is why the president must use his bully pulpit to explain why it is in the vital interests of the United States to defeat the Taliban, destroy al-Qaeda, and establish a free, sovereign Afghanistan that can govern itself and look after its own people. He will be speaking from West Point, and his audience surely knows what is at stake.

Failure to achieve these goals is not an option, for it would be a direct threat to our national well-being. That’s not theory; it’s historical fact. We’ve already walked away from Afghanistan once, in the early 1990s, thinking that what happened there couldn’t possibly hurt us here. We were wrong.

The alternative to victory in Afghanistan is a return to chaos and, quite possibly, genocide. Al-Qaeda and its local Taliban enablers would immediately fill the ensuing power vacuum, turning that benighted land into an apocalyptic failed state. This would recreate the exact conditions that produced the 9/11 attacks.

Only this time, things could be worse. We could witness a regional conflagration that quickly turned nuclear and went global. Afghanistan borders on Pakistan, a nuclear nation with many Taliban sympathizers (especially among its ethnic Pashtuns).

A Taliban-dominated Afghanistan could easily inject further instability in Pakistan, strengthening extremist forces in the region that also threaten India. The likelihood of war between India and Pakistan — a war that could potentially go nuclear — would rise significantly. Remember, these two countries have already fought three wars since the partition of British India in 1947, and enmity between the two still abounds.

These are the stakes in Afghanistan. Defeating our enemies there and leaving behind a stable state is a national-security priority. President Obama must make this case without hesitation, obfuscation, or qualification.

Unfortunately, the president has thus far displayed irresoluteness on this key security issue. The publicized uncertainty in his decision-making augurs ill on several fronts.

It is inexcusable that he has taken so long to reach a decision on troop levels. The man requesting more troops, Gen. Stanley McChrystal, was personally chosen by President Obama to lead the operation in Afghanistan. His request for troops was based on months of quite deliberative strategy reviews.

Yet the president has indulged in many more months of “review,” involving more and more people with less and less knowledge of the military situation in Afghanistan. The needless delay has put the mission in graver jeopardy and left the lives of American servicemen unnecessarily at risk.

Then there’s the matter of troop strength. News reports based on White House leaks indicate that the president will tell the nation he is sending somewhere in the neighborhood of 35,000 troops, possibly arguing that these are only 5,000 fewer than General McChrystal asked for.

The political calculation is that sending a smaller number of troops would pacify the Democratic party’s leftist base, which wants an immediate U.S. withdrawal. But the military calculation should take precedence.

The New York Times has revealed that Gen. McChrystal’s original assessment called for an additional 60,000 to 80,000 troops to maximize the chance of success. A decision to send in less than that incurs a greater risk of failure.

General McChrystal was quickly muzzled by a White House apprehensive that he was asking for bigger numbers of troops. Some Democrats even called for the president to fire his general if McChrystal continued speaking to the press. None of this, however, should make us forget that the president reportedly will announce that he will send in fewer troops than his commander on the ground says is needed to achieve maximum success.

Whatever troop level is announced, the president should refrain from confusing the nation with talk of “an exit strategy.” We all want our troops to come home. But the president’s strategy should focus on creating the conditions for that outcome — which is a stable Afghan government capable of preventing Taliban and al-Qaeda safe havens.

Talking about “exits” rather than “victory” sends the wrong message to our enemies. It tells them that we are not in the war to win, and that we may very well throw in the towel if they offer more resistance.

One final point: Yes, it will be extremely disappointing if the president fails to commit all the troops requested by the field command. But even if that happens, we should be wary of the argument that “if we’re not in to win, we should pull out now.” That’s a false choice, because a pullout is a de facto defeat.

This war is winnable if the president will deploy the resources and strategy to achieve victory. If he does not, then a choice will indeed have been made. Defeat carries dire consequences.

Cross-posted at The Corner.