How can anyone argue that the world did not change on September 11? This week, the fifth anniversary, reflections suggest that the world has changed in many ways.

Watching the memorial services and listening again to recordings of the distress calls from that fateful day makes you think again about the enemy we face. Mostly, perhaps, the horrific events of that day have forced us to think about what kind of world we want to live in, and what we will do to defend our way of life in a liberal democracy. 

An article in Foreign Policy magazine, "The Day Nothing Much Changed" by managing editor William Dobson argues sophistically that the world did not change on September 11, that in fact it changed 15 years ago with the end of the Cold War. That made the United States the sole remaining superpower, and, at the same time, this kind of power made the United States a target to a degree it had never been before, not only for anti-Americanism but for terrorism as well. 

"The tragedy of 9/11," writes Mr. Dobson, "was a manifestation of a unipolar disorder the world had entered a decade earlier. A day after 9/11, we were still living in the post-Cold War era, we still are today, and that is precisely the problem." As has frequently been done by critics of the United States post-September 11, this line of argument turns the blame for the attack back on the United States and on American power. 

While there is some logic to the argument -- especially given the timing of the initial attack on the New York Trade Towers in 1993, which followed closely on the collapse of the Soviet Union as well as other terrorist attacks that went unaddressed by the Clinton administration throughout the 1990s -- it is clear that by the sheer magnitude of the September 11 attacks much has changed in the world. Not only is there the way it changed the American psyche, but also the way the Bush administration has looked at the world. Let us look at the ways: 

• The innocent optimism about the post-Cold War World that was summarized by Frances Fukuyama as "the end of history" went up in the clouds of dust from the falling Twin Towers. Communism might be gone, but its place has been taken by another ideology in the form of radical Islam. History is back with a vengeance; yet we are no longer looking to explain the struggle we are in by reference to Hegel and Marx. In the past five years, it has been necessary to look for explanations of recent events in the Koran to recent and its commentaries. 
• The impossible suddenly became thinkable on September 11. Not even Hollywood had dreamed up movie plots as horrendous as the terrorist attack on major landmarks and government buildings with fully fueled aircraft. In a world where this is possible, just about anything can happen. Though daily life has returned more or less than normal, the subtext is that no idea is too crazy for someone to attempt its execution. Today, travelers between the United States and Britain patiently wait to be searched, drink baby formula and throw away their lipstick and diaper cream to prevent anyone from mixing a bomb on board a trans-Atlantic flight. As surreal as this could appear, no one demurs. 
• The struggle within Islam was made evident by the attacks. Radical, violent fundamentalists like Osama bin Laden and the suicide bombers he dispatched to die on his behalf have a growing appeal among young Muslims looking for causes and certainties. The struggle that we sometimes between two civilizations is as much a struggle within the world of Islam. Our future depends equally on the outcome. 
• American foreign policy changed radically on September 11, as the perception of a threat from abroad changed. While the Bush administration had come into office with a fairly humble and limited foreign policy agenda, September 11 changed all that. Mr. Bush became in one instant a foreign policy president who presided over the first chapter in the global war on terror, the wars in Afghanistan and Iraq. 

Occupants of the White House -- Democrats and Republicans -- cannot afford to be complacent about the threat of terrorism. Five years on, we are still struggling through its long shadow.

Helle Dale is director of the Douglas and Sarah Allison Center for Foreign Policy Studies at the Heritage Foundation.