It is understandable, given the chaos surrounding the U.S. exit from Afghanistan — especially the murder of the 13 U.S. troops — that our attention has been focused on the ending of America’s longest war.

The statistics are sobering. Length of the war: 20 years. Total number of U.S. fatalities: 2,461. U.S. wounded: more than 20,000. Afghan fatalities: more than 70,000. Cost of the war: $2.3 trillion.

Major goal achieved: The Taliban were removed in late 2001 as the head of the Afghan government, denying a base of operations to al-Qaida, the terrorist group responsible for the 9/11 attacks in New York City and Washington, D.C. Since 9/11, there have been no similar attacks by Islamist radicals in the United States that were planned or executed from Afghanistan.

Goal not achieved: The conversion of Afghanistan from a congeries of independent tribes into a nation-state able to manage its own internal and external security without interference by outside powers.

Despite the remarkable airlift of an estimated 130,000 Americans, allies and Afghan friends, there are between 200 and 1,000 U.S. citizens remaining in the country who want to get out and cannot.

There are critical lessons to be learned from the Afghan War.

One, we should only go to war with clearly defined realistic objectives and an exit strategy. Heritage’s long-standing foreign policy expert Kim Holmes has outlined our vital interests that would justify war:

“Protecting American territory, sea lanes, and air space. Preventing a major power from controlling Europe, East Asia or the Persian Gulf. Ensuring U.S. access to world resources. Expanding free trade throughout the world. Protecting Americans against threats to their lives and well-being.”

It is the last condition that justified the Afghan War: which, in fact, we won in a matter of months and with the loss of a handful of soldiers. (The Taliban lost control of Afghanistan by early December.) Not on the above list: nation-building.

Two, we must accept that we live in a dangerous, violent world that requires us to maintain a strong national defense to defend our vital interests.

Three, we should remember that we won the initial stage of the Afghan War but lost the peace because of the withdrawal of just a small U.S. force and air support. This force was enough to keep the Taliban from seizing and holding a single provincial capital in almost 20 years.

Four, we should look back and reflect how Americans came together after 9/11.

Immediately following the terrorist attacks, there was a mighty surge in military enlistments by teens and young adults so often dismissed as spoiled and self-absorbed. Thousands of American lined up to give blood.

Attendance at churches, synagogues, and mosques doubled. Among the messages from religious leaders, the words of Pope John Paul II stand out: “Even if the forces of darkness appear to prevail, those who believe in God know that evil and death do not have the final word.”

Americans did not crack and come apart. They stood strong on a foundation the Founding Fathers built two-and-a-half centuries ago: a unique mix of political and economic liberty.

Is such brotherhood possible today? It is: if we draw on the wisdom and the resolve of the past. If we abandon the rigid mindset of red and blue, black and white, rich and poor. If we reaffirm our commitment to a united America.

Lee Edwards, Ph.D., is the Distinguished Fellow in Conservative Thought in the B. Kenneth Simon Center for Principles and Politics at The Heritage Foundation.