After the horror, the rage and the heartbreak, comes the cold realisation that the Republic of Ireland faces the hardest decisions it has had to make in the entire course of the Troubles. Ambiguities are gone. There is no room for the sneaking regard. There is no point in thinking about political formulae or clever diplomatic manoeuvres. All the old rhetoric of sovereignty and war and imperialism has been blown asunder.
Now, there is just this question: what does a democratic society do when a murder gang declares all-out war on its most basic principles? How, in other words, does a civilised republic react when it offers people an honourable way out of a long, terrible period of violence and they respond by upping the ante and throwing a pile of severed limbs on to the negotiating table?
Perhaps the Republic should have spent more time preparing its answers to that question before last Saturday afternoon. Logic is a hard word to use in the aftermath of such an atrocity, but the logic of the political situation on this island was that the diehards would go for broke.
By the autumn, if the peace process remains on track, a series of profound and perhaps irreversible political changes will begin to take effect in Northern Ireland. An overwhelming majority of the people of the island, North and South, has given its free consent to those changes.
For those who can't wean themselves off the steady diet of blood and fanaticism they have consumed for more than a generation, it is a case of now or never.
In that sense, Omagh is stunning, revolting, but not surprising. The carnage is unimaginable but not unpredictable. Do we not know from the history of violent Irish republicanism in the 20th century that it has always had an unnappeasable strain? Has it not had at its heart an element of sheer nihilism, a twisted mentality that will accept nothing short of the impossible, and that therefore is reduced to the pursuit of violence for its own sake?
And how could we have forgotten that, as well as some perverted idealists and some victims of political circumstance, the IRA that has conducted its armed struggle over the last 30 years clearly had within its ranks a hard core of sadists and psychopaths?
It was always obvious that, for all the political complexities and all the force of history, there were also "republicans" who were getting serious enjoyment from picking off the members of Protestant Border families one by one, from strapping a man into his lorry, loading it with explosives and making him drive it towards an army checkpoint, from sawing through some kid's kneecap with a Black and Decker drill.
For such people, politics was just an opportunity and an excuse, and political change merely means having to change the excuse.
Over the last two years, we've grown used to using a language full of ambiguities. We've learned to look for the rational, political side of those who commit and condone mass murder. We've learned to put to the back of our minds the images of what it is that "militant republicans" and "militant loyalists" actually do: rip and tear and shatter and shred the bodies of children, women and men.
We've allowed our minds to wander from the true nature of their heroics, captured in all its righteous savagery by James Simmons in his poem, From The Irish:
Familiar things you might brush against or tread
upon in the daily round, were glistening red
with the slaughter the hero caused, though he had gone.
By proxy his bomb exploded, his valour shone.
It's been hard to take our eyes off the mesmerising transformations of terrorists into statesmen. Watching the painful emergence into democratic politics of many of those within the IRA, the UDA, and the UVF who have, at one time or another, been up to their necks in the business of mass slaughter, it has been too easy to forget those for whom such a transformation is neither possible nor desirable.
Such people present a democracy with a ferocious challenge. An elected government cannot, in the familiar jargon, "address the causes" of their discontent. It cannot send go-betweens to search out the opportunities for constructive dialogue. It can't cajole them with handshakes and forums or tempt them with trips to the White House. It can't, in other words, do what elected governments are good at - practise politics.
All it can do is find the killers, lock them up and break their organisation with all the speed, efficiency and ruthlessness it can muster while trying to do so without destroying the very democracy the State is seeking to preserve.
For the stark truth is that just as we have learned that there is no security solution to the old violence in Northern Ireland, only a political one, now we have to face the fact that there is no political solution to the new violence, only a security solution.
We might have hoped that the first test of the new political arrangements emerging from the Belfast Agreement would have been the creation of jobs or the dismantling of sectarian education, or the tackling of poverty. Instead, grimly, the first test is the willingness and ability of all democrats on both parts of the island to fight for democracy itself.