For Americans growing up in the 1950s and 1960s, internal military danger was a familiar issue. Although there was no equivalent attack on the nation's soil in these decades, preparations for warfare were commonplace.

During the Cold War, the federal Civil Defense Administration and Atomic Energy Commission conducted all sorts of drills to prepare the nation for an atomic and nuclear war. School children experienced "duck and cover'' drills starting in 1950: Teachers would yell "Duck!'' at unexpected moments and children would kneel by their desks and cover their necks and heads. In 1951, the New York City school system started distributing dog tags to children; other cities followed. Although the purpose was never explained to the children, government officials wanted to be able to identify their bodily remains after a war. The national government also distributed cartoon books featuring Bert the Turtle, who instructed kids about how to act in case of an atomic bomb.

Wealthier Americans built private bomb shelters in their backyards. Life magazine, as the historian Elaine Tyler May once found, even featured a young couple who spent their honeymoon in a bomb shelter. Families watching television saw congressional investigations about alleged conspiracies to undermine the security of the nation.

Besides civil defense, younger Americans of the Cold War era actually experienced war. Born during or shortly after World War II, they and their families understood the threat of global military campaigns. During the Cuban Missile Crisis in October 1962, moreover, the nation was on the brink of nuclear disaster for 13 days. Although the Vietnam War did not bring combat inside our borders, millions of Americans either had relatives in combat or served in the effort themselves.

For Americans born in the 1980s and 1990s -- those who will be the next generation of economic and political leaders -- internal security has been taken for granted. As the Cold War waned and knowledge about the destructiveness of nuclear weapons became widespread, civil defense campaigns were abandoned. Thankfully, our generation was able to focus on our studies, love lives and decisions about what cable channel to watch, rather than thinking about entering a war.

Even the wars that the United States has been involved in since Vietnam, such as the Gulf War, appeared bloodless from our perspective.

Most Americans who lived through the Gulf War visualize images resembling a video game rather than the carnage of D-Day. Of course, many lives were lost and harmed in the Middle East, and severe damage was inflicted on Iraq, but these were not experiences that most younger Americans felt first-hand.

While a sea change started in the 1990s, with the first bombing at the World Trade Center and the bombing of a government building in Oklahoma City, never has the threat, and subsequent sense of danger, been as great in scale or scope as after the events of Sept. 11.

The innocence of our generation is lost forever. We have entered a new Cold War, although this time the enemy is not as clear, the political objectives are much more vague, and the actual military threats that we face remain something of a mystery. Like the Cold War, this will be an ongoing battle and the risks will be high.

These historical experiences can never be erased. Like the baby boomers who proceeded us, we will now live with the memories and fears that come from being part of an international community.

We will have to learn the costs and burdens that come from surviving in such a world.

At the same time, we will have to be extremely cautious not to repeat the errors that previous generations have made, often against innocent neighbors within our borders, in times of crisis.

But for now, my generation will have to duck and cover, accepting the terror of modern warfare.

Julian E. Zelizer, a member of Generation X, is associate professor of history and public policy at the University at Albany and author of "Taxing America'' (Cambridge University Press).