If Democrats needed any reminding, Sunday's new terror alert underscored why John Kerry's managers made the right choice in turning his convention into an ode to patriotism, toughness and the martial virtues.

Homeland Security Secretary Tom Ridge's announcement of a threat against financial institutions here, in New York City and in Newark gave Kerry early warning that he will be campaigning in circumstances over which he will have limited control.

The power to call alerts rests in the hands of the administration he is trying to unseat. Kerry will be forced again and again to react to his opponents' warnings and initiatives -- including President Bush's acceptance on Monday of the Sept. 11 commission's recommendation for a new intelligence czar and a counterterrorism center.

Last week's Boston convention can thus be seen as a shrewd exercise in preemptive political warfare. If there is one certainty about the coming months, it is that Bush will push the campaign back to the war on terrorism and the immediacy of the terrorist threat. Variations on the classic incumbent's slogan in situations of peril, "Don't change horses in midstream," were evoked by Abraham Lincoln and Franklin D. Roosevelt. It will be Bush's subliminal message this year.

Kerry's response stands in sharp contrast to the approach taken by Democrats who tried to push the 2002 midterm election campaign away from national security concerns. This year's Boston Democrats -- that's the Boston patriots, not the Boston liberals -- knew better. Not content with arguing that Kerry was tough enough to handle this moment's threats, Democrats made the case that Kerry was actually tougher than Bush. Kerry had seen battle, knew what war was like and understood in a way his opponent could not the struggles and sacrifices of those facing enemy fire.

And beneath the obvious message was a more subtle theme. This was the convention in which Democrats would redefine the meaning of the 1960s. The Vietnam War, which nearly destroyed the party in 1968, became the unifying event of 2004. After 36 years, Democrats finally made their peace with Vietnam and with the decade that tore their party apart.

Until now political conservatives have successfully imposed their own understanding of what "the '60s" were about -- a time of drugs, sexual license, rebellion in the streets and American self-hatred.

In one of the boldest but least-noticed passages in his acceptance speech, Kerry flatly rejected that definition.

"My parents inspired me to serve," Kerry declared, "and when I was a junior in high school, John Kennedy called my generation to service. It was the beginning of a great journey -- a time to march for civil rights, for voting rights, for the environment, for women and for peace. We believed we could change the world. And you know what? We did. But we're not finished."

Note, first, the unapologetic use of the word "we." Kerry does not shrink from being cast as a 1960s figure. He welcomes it. But note also that Kerry's 1960s are defined not by the despair that took hold at the end of the decade, as the Vietnam War was grinding on to its dispiriting end, but by the optimism of early and mid-decade. This was the 1960s of John F. Kennedy -- and, yes, of Lyndon B. Johnson before his presidency was ensnared in Southeast Asia. It was an era in which idealism forged genuine achievements that the nation does not wish to roll back. Kerry is insisting that in so many ways, the 1960s were good for our nation.

It is not surprising that the man who would be the first Vietnam veteran in the White House has taken on the task of binding the wounds inflicted by that war. Kerry, a product of the 1960s, insists on an understanding of that era that would allow the nation to pick up on the best of the decade's legacy while leaving aside the rancor with which it closed.

Is there a political purpose to all this? Of course. In the short run, Kerry's Vietnam service allows him to stand apart not only from the incumbent he is trying to defeat but also from Bill Clinton, whose decision to avoid the draft gave conservatives ample opportunity to keep alive the divisions created during the '60s. And, yes, the military testimonials will make it hard for Republicans to label Kerry a wimp whenever the terror alerts are sounded.

For the longer haul, Kerry is sending a signal that he is tired of seeing his party walk away from the best parts of its inheritance. Conservatives revere their own tradition, and good for them. It's about time that liberals did the same.

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