The discovery of another cross-border landmine reminds us of this central truth: the Border exists in its current form largely because constitutional Irish republicans prefer it that way.
For the relationship between the border and the republican mindset is one of the defining and paradoxical features of Irish life. And whereas the entire republican family on the one hand deplore the Border, on the other hand, all of them cherish it with differing degrees of geographical, emotional and legal enthusiasm.
This enthusiasm comes in different ways. Thirty years ago, after the monstrous atrocities at Mullaghmore and Warrenpoint, and in the absence of any proper aerial surveillance capacity by the Republic's defence forces, Jack Lynch agreed to proposals from London for limited overflights by British aircraft in hot pursuit of terrorists. This - not the wretched security failures of which Mullaghmore and Warrenpoint were merely part - was why Jack Lynch was promptly overthrown in a coup organised by Charles Haughey.
Twelve perfectly splendid Haugheyite years were to follow.
The continuum of 'republicanism' - though our use of this word has no intellectual connection at all with its US or French usages - allows members of the republican tribe to slide the interpretation of the word 'border' across a spectrum of meanings.
This varies from it being an insupportable insult to the land of the Annals of the Four Masters, to being able to enjoy cheap shopping in Sainsburys with a clear conscience. But the Border is also a plaything with which to torment the British. So why would the Real IRA - or whatever is the heir to Liam Lynch, Sean Ryan, Sean MacStiofain, Daithi O'Conail, Brian Keenan et alia - not use cross-border murder techniques?
They know this simple truth. The authorities in the Republic would prefer the legal integrity of the Border be retained, to the last phoney inch of its existence, rather than the integrity of the life and body of a member of the Northern security forces be similarly protected, by a complete removal of any protection that border confers to a cross-border terrorist.
Twenty years of peace-process propitiation has not removed this central truth. A sniper or landmine-operator working cross-border in the Republic against the security forces in the North can still claim the protection of this jurisdiction. And that is barking mad - it is insane; it is infirm and pathetic and deranged and most of all, ladies and gentlemen of the jury, it is wicked.
It confirms that the primary acts of governmental will in the Good Friday Agreement were confined to political settlements, to legalisms and to jurisdictional niceties.
But that joint governmental will did not confirm the right of life to all, to be enforced impartially and instantly by the authorities, with equal vigour, on either side of the border, by whomever, and without jurisdictional let or hindrance.
In other words, there was no Kameradschaft. This was the mythic brotherhood formed when French coalminers tunnelled cross-border to the rescue of German colliers trapped by an underground pit-blast just after the Great War.
And why no Kameradshaft? Because, even though the Border is invisible, it is cherished by those who purport to loathe it. It is ours, a vital part of the identity of this 'Republic'. So, when two British soldiers raiding the Northern side of Slab Murphy's farm strayed feet into Louth to seize two suspected IRA terrorists who had just attacked a British base, whom did gardai arrest and take to Dundalk Garda station?
The Brits of course, not the terrorists. And of outcry from within the Republic, there was, naturally, none.
Many kinds of borderless dispensations have recently been created across our 'border' counties: oncology and casualty and physio units, EU schemes, and cultural and tourism bodies galore.
Meanwhile, the Border is completely invisible to travellers. But where police-kamerad stares at police-kamerad across a frontier that can actually be detected only by satnav-assisted surveyors, why, then it becomes a legal Berlin Wall, protected by the watchtowers of the Republic's laws and by the casuistical vigilance of the Four Courts, wherein squads of theodolite-bearing SCs are poised, exultantly twitching.
In all the breakthroughs of the peace process in the North, one vital element was not achieved. This was the creation of a Border Corps, with one single secretariat, one uniform, one badge, and one over-riding, trans-jurisdictional imperative: the duty to save life.
Such a force could have answered initially to London and Dublin, later to Belfast and Dublin, and its members would have freedom of movement on either side of the border. But, of course, there is no Border Corps, because constitutional 'republicans', if only subconsciously, still cherish the Border. For it is a wound that defines this Republic, an indelible stigma from the crucifixion of Irish sovereignty; and thus those who made that cross - or their servants anyway - must sometimes be made to pay for that crucifixion, even in blood.
Better that, than this Republic forfeit an inch of territorial sovereignty: thus as it was in 1979, so is it still today.
And to judge from the gathering pace of dissident terrorism, now sinisterly echoing the prelude to Omagh, the evil day of cross-border bloodshed is drawing nigh.