We are confident that not only is it winnable, but that we have no choice. We must prevail in Afghanistan.

We went to war there because the 9/11 attacks were a direct consequence of the safe haven given to al Qaeda in that country under the Taliban. We remain at war because a resurgent Taliban, still allied with al Qaeda, is trying to restore its brutal regime and re-establish that country as a terrorist safe haven.

It remains a clear, vital national interest of the United States to prevent this from happening. Yet an increasing number of commentators, including some of the very same individuals who opposed the surge in Iraq and called for withdrawal there, now declare Afghanistan essentially unwinnable. Had their view prevailed with respect to Iraq in 2006 and 2007, the consequences of our failure there would have been catastrophic.

Similarly, the ramifications of an American defeat in Afghanistan would not only be a devastating setback for our nation in what is now the central front in the global war on terror, but would inevitably further destabilize neighboring, nuclear Pakistan. Those who advocate such a course were wrong about Iraq, and they are wrong about Afghanistan.

The growing calls for withdrawal reflect, more than anything, our failure to show progress in the war. After eight years of fighting, the American people see rising casualties and no sign that the tide is turning in our direction.

Their doubts are natural and understandable, and we must respond to them directly and clearly. Our problems in Afghanistan are not because the Taliban are invincible or popular. They are neither. Rather, our problems result from what was, for years, a mismanaged and underresourced war.

Our mistakes are infuriating, but they are also reversible. We traveled to Afghanistan nine months ago and again last month. In the intervening time, a significant shift in our strategic leadership and focus has taken place there.

We have an exceptional new commander on the ground, Gen. Stanley McChrystal, who has begun a top-to-bottom overhaul of all aspects of our war policy and put forward a dramatically new civil-military strategy that clearly identifies failed policies and prioritizes the proven principles of counterinsurgency, including protecting civilians, creating legitimate and effective governance, and boosting economic development. With Gen. McChrystal, together with a new ambassador and a new deputy commander, we believe we now have the team on the ground that can win this war.

However, we need more than the right team and the right strategy. This team must also have the resources it needs to succeed—including a significant increase in U.S. forces.

More troops will not guarantee success in Afghanistan, but a failure to send them is a guarantee of failure. As we saw in Iraq, numbers matter in counterinsurgency. Protecting the population and developing capable indigenous security forces are inherently manpower-intensive endeavors. Moreover, in the absence of basic security, the other crucial components of successful counterinsurgency—fostering the emergence of effective, legitimate government and economic development—simply cannot get off the ground.

We recognize that a decision to increase the number of American troops in Afghanistan will be politically difficult here at home. Some will say we can't afford it. Others will warn the president of "quagmire" and urge him to send either no new forces, or fewer than Gen. McChrystal recommends—perhaps with the promise of "re-evaluating" further deployments later on.

It is precisely this middle path—which the previous administration pursued for too long in Iraq—that is a recipe for quagmire and collapse of political support for the war at home. Mr. Obama was right when he said last year that "You don't muddle through the central front on terror . . . You don't muddle through stamping out the Taliban."

We have reached a seminal moment in our struggle against violent Islamist extremism, and we must commit the "decisive force" that Gen. McChrystal tells us carries the least risk of failure.

We believe that the short-term political reaction from Congress to any increase in troop numbers, no matter how small or large, will be essentially the same. The key question is whether the increase is substantial enough to have a decisive effect on the course of the war within the next 12 to 18 months. If we are to send more of our brave men and women in uniform into harm's way, we should do so in a way that carries the greatest probability of success.

In the interim, the president and his allies—and we count ourselves among them on this issue—must invest significantly greater effort to explain why, as the president recently put it, Afghanistan is a "war of necessity." Additional U.S. resources must be accompanied by significant and meaningful benchmarks that hold the government of Afghanistan and our own government accountable. We must ensure that Afghan leaders are doing their part to combat the corruption and insecurity that undermine the counterinsurgency effort.

We are ready to stand with the president through the tough months ahead, and we believe that strong and steady leadership from the White House can rebuild public support for the war.

The American people also need to hear directly from their commander on the ground. Gen. McChrystal should be called back to Washington to testify before Congress about his new strategy and the resources it will require.

When Gen. David Petraeus testified before Congress in September 2007 about the progress of the surge in Iraq, it allowed everyone to make better informed decisions about our war effort and likelihood of success. As members of the Senate Armed Services Committee, we believe that there should be the same opportunity with regard to Afghanistan this fall. We are confident that, the more Americans hear from Gen. McChrystal and our other military leaders, the more their confidence in the war will be restored.

The U.S. walked away from Afghanistan once before, following the Soviet collapse. The result was 9/11. We must not make that mistake again.

At last, we have the right strategy and the civilian and military leaders on the ground in Afghanistan to carry it out. This is a must-win war. And now is the time to commit the decisive military force necessary to prevail.