“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 as an example for us today.

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.

Millions of dollars were donated to the American Red Cross, the Salvation Army and other welfare organizations. 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.”

Examples of patriotism and sacrifice were everywhere. There was the quiet eloquence of an office worker explaining why he risked his life to help someone buried in the ruble at Ground Zero in lower Manhattan. A New York City fire chief choked back tears as he spoke of a priest friend who disappeared in a cloud of dust as he searched for people to comfort.

There were the passengers on United Airlines 93 who decided to “do something” to stop the terrorists who had seized their plane. Their heroism prevented the hijackers from plunging the jet airliner into the U.S. Capitol.

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.

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A great president has attested to American exceptionalism. President Ronald Reagan declared that what united us far outweighed what divided us. Speaking beneath the Statue of Liberty in New York City Harbor, the president reaffirmed that we are one nation under God. Black and white, he said, we are one nation indivisible. Republican and Democrat, we are all Americans.

“Whatever the trial and travail,” Reagan said, “let us pledge ourselves to each other and to the cause of human freedom—the cause that has given light to this land and hope to the world.”

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.

The lessons we draw from the Afghan War matter—to our allies, to our adversaries, and most of all to the rising generation that needs reassurance that America’s best days are not behind her but yet to come.