Washington DC - This is truly an imperial city, the capital of the global superpower. Except that most of the architecture that seems designed to cow down the visitor and impress with its soaring, imperious lines is from the 19th century or the first half of the 20th. Yet Washington hums with power, even in the blazing heat of their first Independence Day since the 11 September, 2001.

The Mall, a slash of green running from the Houses of Congress to the Potomac River, is, as ever, the setting for planned displays of patriotic exuberance. From the US Capitol, the lone remaining bit of topography here, a sound stage will boom out the musical backdrop to the fireworks ignited at the feet of Abraham Lincoln, or at least outside his neo-classical and oddly moving memorial at the far end of the Mall. The plan is for Americans and visitors to line the parks and streets between the two extremes, taking in both the sound and vision of the Fourth of July, 2002.

The normally open spaces at the centre of the city are blotched with security checkpoints and thousands of men and women in uniform are funnelling us through gaps in hastily erected fences, searching handbags, rucksacks, coolers full of ice and water, even cigarette packets. "Ya can't be too sure," I heard one Howard County Maryland sheriff's deputy tell a tourist from Mexico, who nods his head in vigorous assent. Even this morning, the newspapers were full of vaguely worded yet dire warnings of more terror attacks, and the need for vigilance on America's national day. The mood is odd, light and celebratory yet somehow watchful. People glance about them and talk in hushed tones in the shade.

No one, I repeat, no one, makes jokes about terrorism, attacks on the US, anything like that. This is definitely something different about comedy-obsessed America. I go to a vendor's stall to buy a hat, some protection for my follicularly challenged scalp that's now reddening in the blazing sun. My choices are stark, and all almost militant in their patriotism. There's one saying "US Marines", another with "FBI", the letter embroidered on a camouflage motif. The New York police and fire departments are honoured as well, still powerfully venerated here for their heroics last September. Various other police forces have their logos on baseball hats, along with myriad US government agencies concerned with security. I choose "CIA" on plain navy blue in hopes that somehow choosing the symbol of the agency that got it most wrong is a fashion statement. Later I realise that I also got it wrong.

The most powerful of Washington's memorials and monuments, the low black wall inscribed with the names of slain veterans of Vietnam, is for once not thronged with emotional families and surviving spouses and children. It's always been the most moving moment, for me, of a visit here. The sight of a teenager who probably never knew his father or in rare cases, mother, pointing the name out to his friends, finding it among 58,000 victims of what surely still be one of the world's more pointless wars, never fails to make me weepy, confirmed in my opposition to militarism. You often find yourself in conversation with a Vietnam veteran here, and usually he feels much the same. But today-in the unseasonable heat and the security scare-the crowds are sparse. Just a few people run their fingers over the black marble lists of the fallen, just a few tears sizzle on the pavement below. That morning's New York Times brings dire warnings of another war and I can't help but think about the consequences of a military invasion of Iraq. Will that one require another controversial monument to its fallen? I hope we never find out.

Later, night brings a little relief and the din and delight of the fireworks display. Nothing too fancy, just fountains of light and colour etched against the eastern sky. A lone gunman-mad, apparently, and able in today's America to purchase weapons easily-has been the only incident of violence. Commentators seem quick-too quick-to tell us that it's not related to al Qaeda or terrorism, despite the fact that the attacker was Egyptian and his victims Jewish Americans standing at the check-in counter for the Israeli airline El Al.

It's been a peculiar day and I wonder what lies in store for America, and for all of us, between now and the next Fourth of July.