As we commemorate the 10th anniversary of the September 11 attacks, it has become fashionable in some quarters to characterize the U.S. response to the threat of Islamist terrorism since 9/11 as an overreaction. According to this argument, Americans should look back regretfully on this period in our history as a lost decade in which we succumbed to our fears and exaggerated the dangers we faced, betraying our own best values and exhausting ourselves in the process.

Have we made mistakes since 9/11? Of course — just as every nation, including ours, always has in war. Some of these mistakes are obvious and undeniable: the terrible abuses committed at Abu Ghraib and the broader mismanagement of the Iraq war prior to the 2007 surge under General Petraeus, to name two.

But as we look back over our actions over the past ten years, a lot more went right than wrong. What, then, are the lessons of the 9/11 decade?

The first is that we live in a dangerous world — a reality that remains as urgent and important to recognize today as on that clear Tuesday morning ten years ago.

To be sure, we have made significant gains in our security over the past ten years. Our national security and homeland security institutions have been undergone sweeping reform, including the creation of the Department of Homeland Security and a comprehensive reorganization of our intelligence community. We have also severely weakened al Qaeda’s senior leadership in the tribal areas of Pakistan, killing or capturing many of its senior leaders. Strikingly, al Qaeda and its violent Islamist extremist allies have failed to achieve any significant victories over the past decade.

Yet despite the gains we have made, current geopolitical realities do not justify a sense of complacency or closure about the worldwide war we are in. Violent Islamist extremism is weakened but not vanquished. Although Osama bin Laden is dead and al Qaeda’s core is under unprecedented stress, its regional affiliates are on the rise. Somalia and Yemen today provide terrorist sanctuaries for these groups, and we cannot credibly claim to be on course to shut down either. Until we do, we can expect that attacks will continue to be plotted and launched against us and our allies from both of these countries.

Then there is the government of Iran, the leading state sponsor of Islamist terrorism in the world and the patron of Hamas, Hezbollah and various Iraqi extremist groups, all of whom have American blood on their hands. This is a regime whose nuclear program is speeding forward and whose leaders, it was recently revealed, have for years had their own secret relationship with al Qaeda, facilitating the flow of terrorists and funds across Iranian territory.

In addition to these continuing terrorist threats abroad, we also face a new and ominous danger from homegrown and self-radicalized terrorists here in the United States — “lone wolves” like Major Nidal Hasan, who murdered 13 people in 2009 at Fort Hood, Texas.

In sum, this is not a moment when the United States can unilaterally declare a holiday from history. In order to stay safe at home, the U.S. must remain engaged abroad, and — despite budgetary pressures — make the necessary investments to keep our military and other instruments of national power well-resourced and strong.

In recent weeks, some have urged sharp cuts in defense spending. The fact is, as a matter of math, we will not work our way out of our national indebtedness or create the new jobs we need by decimating our national security budget. But doing so will jeopardize our security here at home, and destabilize the broader international order on which our domestic economy and prosperity depend.

A second important lesson of the 9/11 decade is that, contrary to the pessimism that currently pervades our national mood, America remains remarkably strong and resilient. Our people are still capable of pulling together and achieving things that no other nation in the world can. The remarkable effort to locate and then rid the world of Osama bin Laden, deep within Pakistan, is the latest reminder that almost everyone in history who has bet against the United States has, in the end, lost big.

We should also have renewed faith in the power of our ideals, foremost our belief in the universality of human freedom, and the long-term strategic wisdom of standing up for human rights, democracy, and the rule of law around the world. These values are enshrined in the work of our country’s great universities, including Yale, whose mission for over three centuries has been to advance freedom — of inquiry, thought and expression — and to nurture human potential.

Now, across the Middle East and North Africa, millions of Muslims are rising up and peacefully demanding democracy, dignity, and opportunity. In doing so, they are also rejecting the narrative of al Qaeda. Indeed, the rallying cry of the Arab Spring and its successes thus far are the ultimate repudiation of everything violent Islamist extremism stands for.

A third, and final, lesson of this decade concerns the place of surprise in history — a topic that Yale’s own John Lewis Gaddis has written about thoughtfully. From 9/11 itself to the post-invasion Sunni insurgency in Iraq to the Arab Spring, our national security over the past decade has been profoundly shaped by events that, while understandable and even obvious in hindsight, were largely unforeseen by our best and brightest beforehand. As we look to the future, there is therefore one thing we can predict with absolute certainty: our nation will be surprised once again.

The true test of our national leaders — present and future — is not their ability to perfectly predict what will come next for America but our capacity to remain prepared as best we can for a range of possibilities, and to adapt, as a nation, when the unexpected invariably occurs. In this context, it is especially important to remember, as General Petraeus recently warned, that the wars we are required to fight are rarely the ones we predict or plan for. In this respect, it is encouraging to look back and see how well we have adapted to fight the current war with the Islamist terrorists who attacked us 9/11, which we did not expect. But more work lies ahead, requiring the energy, talent, ingenuity, and commitment of the next generation of leaders — many of whom, no doubt, are today at Yale.

In the wake of the tenth remembrance of 9/11, then, Americans have reason to be proud about all that has been done to protect our homeland and to deny our enemies the victories they have sought, vigilant about the continuing threats to our security, and confident about our future and about ourselves as a nation. Above all, a review of the 9/11 decade affirms why, though we do not know exactly how long this conflict will last or what further turns it may take, we should be certain of how it will end — in the defeat of our enemies and the triumph of our values, with the ideology of Islamist extremism joining fascism and communism on the ash heap of history.

Sen. Joseph Lieberman of Connecticut is a 1964 graduate of Morse College, a 1967 graduate of the Law School and a former Chairman of the News.