Pearl Harbour comes to mind as an event comparable to yesterday's dramatic attacks on the symbols of American power. That was war with another state. What is eerie about these attacks is that no such enemy is immediately identifiable.
This is all the more the case considering the remarkable build-up of military security under the Bush administration, dedicated to creating an invulnerable shield with an anti-missile system to protect against rogue states.
The contrast between that ambition and the expertise with which yesterday's attacks were executed, without countervailing measures being taken, is breathtaking.
If an international terrorist group proves to have been responsible, the question must be asked whether a conventional military response based on a huge expenditure on space-based warfare is the best way to counter it.
If, however, any state is identified as having been responsible - and it is hard to see how the attacks could have been carried out without such resources - then it will be much easier for the administration to justify its defences against rogue states.
It is impossible to say with certainty at this stage how the American people will address these questions in the months to come. Patrick Smyth argues in these pages that it seems inevitable the terrorist threat talked up by many analysts sympathetic to the Bush administration will now be taken much more seriously and used to reinforce the arguments for missile defence.
A rallying-around effect will make it easier for him to win those arguments, especially if it is accompanied by very heavy retaliatory attacks.
There will be huge international sympathy for weeks and months as a result. But much will depend on whether these events reinforce the clear trend to unilateralism in the Bush administration. It has been expressed by serial US abstention from international treaties and conventions, a retreat to hegemony which has arguably reduced American influence.
So argues John Ruggie, a distinguished US international-relations scholar, who has just retired from advising the United Nations Secretary General, Kofi Annan. In an article in the International Herald Tribune yesterday he asked, "Why doesn't US power translate more directly into day-to-day influence" at the UN? He says the withholding of funds owed, and a la carte unilateralism on Kyoto, the Anti-Ballistic Missile Treaty, and the International Criminal Court all play into that reduction.
Certainly that would seem to be the case with the Middle East, so much at the centre of speculation about responsibility for yesterday's attacks.
The close US association with Israel will have reinforced the determination of such groups to attack American targets.
If retaliation leads to war against states accused of bolstering them, we are in for a profoundly unstable period comparable to that which followed the Middle East war of the early 1970s.
The orchestration of international policy towards terrorism will be a test of the Bush administration's approach. There will be much goodwill to co-operate with the US on that, as the Taoiseach made plain yesterday following his meeting with Mr Richard Haass, the US adviser on Irish affairs. Ireland's chairmanship of the UN Security Council next month will put this State at the centre of the international consequences of yesterday's events.
But over the months to come, the tensions that have become so obvious between the US and its European allies will re-emerge if a unilateral attitude is taken.
As Mr Ruggie says, this will be up to the administration to determine. Accompanying this fallout will be the economic consequences of the devastation visited on New York yesterday.
The immediate economic effects on share markets, the flight into cash and out of the dollar and the sharp rise in the dollar all illustrate these realities. So does the price of oil.
And it is hard to see how US consumer confidence can ride out the crisis, as many Irish and European economic commentators have hoped.
This could therefore be a real turning point in international economic affairs as well as a major event in international politics.