Put most bluntly, an American age of innocence ended this week. True, that innocence has always been something of a fiction in a country which needs no reminding of its history of racial tensions, appalling crimes and presidential assassinations, and which boasts a murder rate 15 times that of Britain. But deliberate random terror for political ends has always belonged somewhere else - Belfast, Beirut, Buenos Aires, Algeria - not in a humdrum, windswept cowtown bang in the middle of the continental USA, about as far as it is possible to get from the great forces of world history.
Whoever planted the bomb that tore apart the Alfred P Murrah federal building struck at a reality beyond the fractured myth: the reality of middle America, with its unchanging rhythms, eternal certainties and bedrock optimism. These are now fractured, possibly for ever. That a federal building was chosen indicates that the bombers' target was the US government itself. But not a building in Washington, the seat of that government, or a showcase city such as New York, Los Angeles or Chicago - great metropolises where such outrages, however regrettably, are bound occasionally to occur. Instead, fiendishly, they chose an ordinary city, in the American equivalent of nowhere.
Thus the shock, tangible and all-pervasive, across the entire country. How could something so senseless, so utterly evil, happen in a place like Oklahoma? Nothing in the American experience compares with it: not the massacre of the marines in Beirut in 1983: they were, after all, military personnel on service in a dangerous foreign country. As many civilians died on Pan Am flight 103 over Scotland in December 1988 - but aeroplanes are always vulnerable. But not Oklahoma City, a quiet, unchallenged citadel of the American way of life.
Once the shock subsides, however, America's reaction will be savage. When President Clinton appeared in the White House press room to denounce the deed, there was a hard, cold glint in his eye, quite unknown for this most affable and emollient of presidents. The Attorney General, Janet Reno, was glacial as she announced that the federal government, which has not executed anyone since 1963, would seek the death penalty against those responsible.
And she had little choice. Even before this unprecedented barbarism, crime has been the obsession of the American public. Should it prove that the Oklahoma City attack was carried out by Americans for American purposes, pressure on Congress to pass yet more severe "law and order" measures will only grow. If a Middle Eastern connection emerges, the consequences will be far uglier. Should an Arab government be shown to have been involved, military retaliation is all but certain.
Let it not be forgotten that America is not only the land of baseball and apple pie, where the Oklahoma corn grows as high as an elephant's eye. It is also the country which in 1927 executed the Italian-born anarchists Sacco and Vanzetti, which without compunction imprisoned Japanese-Americans during the Second World War, and which was mesmerised by the anti-Communist witch-hunting of Senator Joseph McCarthy.
As the present clamour for capital punishment testifies, the public mood is for vengeance; swift, sure and absolute. If an Arab connection is established, then the lot of Arab-Americans will not be easy, to put it mildly, at a moment when an anti-immigration campaign is growing, from California to Florida. Small wonder many thoughtful Americans are quietly hoping against hope that this car-bombing, for all its foreign hallmarks, is a purely domestic affair, an especially terrible piece of what is nevertheless a recognisable mosaic.
But most early signs suggest otherwise. And what is truly astonishing is not that an outrage of this dimension has happened here, but that it had not happened far earlier. In the Middle East, America has been stockpiling grudges for the best part of half a century. Its very nature as a country of immigrants means that virtually every foreign people or government which feels aggrieved by US behaviour has a potential fifth column inside the country. Thus it seems to have been with the 1993 World Trade Center bombing, where the alleged mastermind arrived from abroad, devised the plan, and then departed to leave local sympathisers to carry it out. So it may have been in Oklahoma City. And yet, thanks probably to a mixture of good police work and old-fashioned luck, 1994 saw not a single terrorist incident on US territory.
Inevitably, there will be pressure to tighten border controls. Like Britain and unlike most nations of continental Europe, the US operates tight controls at its frontiers. Within them, however, there is no system of ID cards. Nothing is easier than simply to disappear in a vast country whose essence is mobility. And all the scrutiny of arriving visitors at airports, and the strict patrols along the border with Mexico, obscures the reality of thousands of miles of coastline and an undefended and largely unmonitored frontier with Canada across which criminals can and do slip with impunity.
Security precautions, too, will undoubtedly tighten. The US president is already the most closely guarded leader in the Western world; protection will surely increase at lower rungs on the official ladder. Controls at federal buildings, of those who go in and out and of suspect packages, will also increase. But as Britain, Spain and Israel have long since learnt, the really determined terrorist is impossible to stop. The air is full of talk about super-sophisticated new technology from the FBI and other agencies; but all the electronic wizardry at America's disposal was unable to prevent a man crashing his Cessna light aircraft into the south wall of the White House last September: what if it had been an F-15 loaded with bombs?
True security would require police-state curbs that Americans could not tolerate - less even than Britons, Spaniards and Israelis. Previously, the US has never even addressed that problem. Now, it must. Oklahoma City has proved that everyone is vulnerable, that no public building, movie house or shopping mall in the country is safe. Americans must develop their own "culture of awareness", grafting a consciousness that terrorism exists on to the way they go about their daily business. Such is the loss of innocence, not a retreat into an isolationism that has been proved illusory, but an entry into the real world, where appalling, inexplicable things do happen.