Eleven years after the fateful events of Sept. 11, 2001, Americans still pause to remember the tragedy that befell the nation that day. In the days and weeks after the worst terrorist attack on U.S. soil in history, Americans came together in shock, grief and outrage over the murder of 3,000 of our fellow citizens. Even as we mourned our dead, we celebrated the heroism of the police, firefighters and emergency medical personnel who risked their lives to save others — the highest expression of our national identity. In our determination to defeat those who attacked us and bring them to justice — and moreover, to preserve America's values in the face of those who would destroy them — Sept. 11 united Americans in a way we rarely have experienced before or since.

When, three days after the attack, President George W. Bush climbed on the rubble where the Twin Towers of New York's World Trade Center had stood, it was out of that sense of shared purpose that he spoke, reminding us that we were all Americans, one nation, one people with a common destiny joined in a common cause. And the country rallied behind him. It was a time when we truly felt that we were all in it together, citizens of a great and diverse nation whose differences were far less important than the values that bound us together.

But that unity was short lived, sadly, because all too soon it was exploited for political purposes and partisan gain. Americans supported the overthrow of the Taliban who had provided a safe haven in Afghanistan for Osama bin Laden and his al-Qaida conspirators. But they were less certain about reports of secret CIA prisons and the Guantanamo Bay detention center where captured militants were tortured to extract information, and many were skeptical of the vast expansion of government surveillance of U.S. citizens in the name of a war on terror. Those anxieties came to a head with the invasion of Iraq two years later, a conflict that was begun on the basis of faulty intelligence that Saddam Hussein was developing weapons of mass destruction and false assertions by the administration that Iraq conspired with the terrorists who plotted the 9/11 attacks.

The Iraq war was a gigantic breach of faith between Americans and their government, and it broke what was left of the unity of purpose among Americans after Sept. 11. The country has never been able to recover it. The war re-opened all the fractures, petty resentment and mistrust between the two political parties, between conservatives and liberals, between red states and blue. In the nine years since the Iraq invasion, those divisions have only grown wider despite the earnest efforts of two presidents to heal the rift.

Today, as a sharply polarized nation looks forward to another presidential election in November, it's worth recalling the sense of common purpose and shared sacrifice Americans felt when they came together to confront the challenge posed by the attacks of Sept. 11, 2001. The nation rallied as one against the common foe because the attack on America made us all realize how dependent we are on each other and how we stand or fall together as a nation.

Mr. Bush and other political leaders fostered that sense of unity in the days after Sept. 11, but they did not create it. Likewise, we do not now need Barack Obama or Mitt Romney to unite us. That power rests within us all, and it can only be denied if we allow it. Our national motto, E pluribus unum — out of many, one — still sums up the best in our national character. On this day let us remember that out of our many we have become one, and that we can do so again.