At this moment sixteen years ago, the world changed on a beautiful Tuesday morning in New York and Washington, DC. Radical Islamic terrorists seized four passenger flights and used them as guided missiles aimed at the heart of American political and economic power. Three of the planes found their targets at the World Trade Center and the Pentagon, killing nearly three thousand people in the process. The fourth never hit its intended target after passengers on United 93 fought the first battle of the new war and brought the plane down in Shanksville, Pennsylvania, saving an untold number of lives.

Each year we mark 9/11, as both a time of mourning and a time of remembrance, as Patriots Day. We remember especially the patriots who responded that day — the heroes of Flight 93 called to action without any preparation at all, for instance. We remember the men and women who ran into the collapsing buildings to help more run out to safety, and who gave their lives for their fellow Americans. We also remember the men and women who rushed to their nation’s aid to enlist in the military and serve in the cause of liberty and freedom from terror, both at that time and in the sixteen years since. We remember these patriots with a sense of gratitude, and also a sense of humility.

Patriots Day should also mark a day of renewed purpose. The fight against radical Islamic terrorism goes on, and will not end soon, but must be fought tenaciously and wisely. The same ideology seeks to destroy freedom and liberty to this day, and the years of ISIS have demonstrated exactly what they have in mind if they ever gain power. But we also should recommit ourselves to the ideals of freedom and liberty that are under attack — the freedom of religious expression, free speech, free assembly, and the rule of law by republican self-governance. Radical Islamists are not the only entities that seek to impose their will by violence and oppression.

The heroes of 9/11 remind us of both the fragility of liberty, and its strength among people who know freedom and despise totalitarianism. If we are indeed only one generation from losing liberty, then let 9/11 and Patriots Day renew our commitment to it, as well as our defiance of those who think they can take freedom and liberty from us.