ST. LOUIS -- Sept. 11, 2001 was the most consequential geopolitical event since the dropping of the atom bomb. Many political, cultural and social assumptions were abruptly transformed. Unabashed patriotism of a type not seen since World War II became fashionable.

But the most immediate and sweeping changes were to our assumptions of invulnerability. For the first time since the earliest days of our nation's history, mainland America was attacked from abroad. We lost hundreds of thousands of lives in two world wars, but we always believed that our mainland was safe from attack.

That changed on Sept. 11.

A pervasive sense of vulnerability made people more receptive to security needs and less concerned about privacy and civil liberties. Attorney General John Ashcroft was able to single-handedly amend the Bill of Rights.

"Unlawful combatant" became code words describing hundreds of people deprived of any constitutional rights and incarcerated indefinitely. The names of aliens in custody would be kept secret, and their deportation hearings likewise would be conducted in secrecy. American citizens who were classified as "unlawful combatants" would have their names made public but would be given no other legal protection.

After Sept. 11, we could no longer afford the luxury of complacency. The prestige and influence of the U.S. military has skyrocketed. America is now more likely than at any time since the middle of the last century to intervene militarily overseas when we perceive that our interests are threatened.

Prior to Sept. 11, we had engaged in minor pre-emptive forays in our hemisphere, such as Nicaragua, Haiti, Dominican Republic, Panama and Mexico. We talked softly and carried a big stick in the Caribbean, where we deemed ourselves to have authority under the Monroe Doctrine. Never before, however, had we claimed the right to start a war with a far-off nation which had a significant military force fortified with chemical and biological weapons and a threatening reputation to use them.

In the past, we had deluded ourselves with the notion that we had steadfast allies in the Persian Gulf where their oil and our money constituted a permanent linkage of friend with friend. We wanted to believe that the Israelis and Palestinians, as well as the Islamic world at large, viewed us as the honest broker that some day could facilitate an end to the Mideast conflict.

We were stunned as the depth of anti-Americanism in the Islamic world became strikingly clear. More than one magazine cover story read, "Why Do They Hate Us?" We reciprocated by becoming pervasively suspicious of all Muslims, including the six million living in our own nation.

There now appears to be a deep cultural and political breach between the Judeo-Christian world and the Islamic world. When we invade Iraq, that breach will widen and will create our most worrisome foreign-policy concern in these early days of the 21st century.

Under President George W. Bush, we will spend billions of dollars in Afghanistan building a nation where none can exist and billions of dollars in Iraq, where our invasion may cause a nation to fall apart.

The world that existed on Sept. 10, 2001, has disappeared forever. We are a different country in a different world, and we now face different and difficult challenges. The test of a country's character is measured in times of adversity. We will make some mistakes along the way, but we will pass the difficult test that Sept. 11 has imposed on us.

Thomas F. Eagleton is a former U.S. senator from Missouri who currently practices law in St. Louis.