History shows what mayhem follows when wartime combatants are cut loose in peacetime - unappreciated, unemployed and angry.
This is at the root of much of the misery afflicting our planet, with examples spanning the notorious James Younger gang through to the insurgents of the so-called Isis (Islamic State).
The James and Younger boys formed their gang during the economic and social chaos following the American Civil War.
As teens they'd seen love-ones murdered and then became Confederate irregulars, learning for themselves how to intimidate, bully and kill.
Confederate irregulars were no better or worse than their Union counterparts in this regard. On both sides, a rabble of un-uniformed thugs seemed more intent on terrorising and even massacring soft-target civilian populations than fighting one another.
And in this respect - even if the nuances of Middle East geopolitics are too much for the supporters of Donald Trump to grasp - they should take a timely lesson from the infamous Jesse James.
This outlaw's exploits in robbing banks and trains should not belie the fact that he was a casual and vindictive murderer.
Despite manipulating the media to create a folk-hero image and status, James was just another damaged human being. A man out to punish the world for the violence he'd suffered as a child.
Trump may now be advocating the torture of terrorists - but leaving aside increasing the risk to his country's own soldiers - this will never stop terrorism.
Yes, some jihadists are implacable rich kids who, following radicalisation, leave behind middle-class lifestyles to pursue foreign wars. But many are from the other side of the tracks - young men with little else going for them.
They have no prospect of education or a job; they've been raised amid violence, so they pick up a gun.
And notwithstanding the religious fanaticism, which of course Isis does not hold in common with those old civil war militias, there are similarities worth considering.
Isis was formed out of a war. It is substantially led by former members of Saddam Hussein's Baath Party, ex-Republican Guards who were deliberately given little chance of employment in the new Iraq.
Many commentators point to the Government of George W. Bush as a primary influence in the formation of this insurgency.
In their overweening arrogance, it was Bush, Cheney and Rumsfeld, who - while lacking any coherent plan to maintain the "peace" which would surely follow - orchestrated the 2003 invasion of Iraq.
Many Isis commanders are former Republican Guards, men not particularly religious, being motivated primarily by sectarian and nationalistic goals.
It's said they are happy to put the "useful idiots" under their command - meaning the religious fanatics from overseas - into front lines to do the fighting.
There's a simple lesson, which the neoconservatives would have known had they studied history: after war has ceased and peace has been declared, you either rehabilitate the former combatants into productive civilian life, or risk mayhem.
Failure to provide decent peacetime lives for regular soldiers - men trained to kill people and destroy property by the way - is risky. Failure to do so for likes of Republican Guards, whose modus operandi used to include torture, rape, public executions and use of a kind of Stalinist secret police - is 10 times more risky.
Of course, former combatants in all wars suffer from physical and mental trauma that reduces empathy.
But while post-traumatic stress can lead to drug and alcohol abuse, mental health problems and domestic violence, these can prove least of our worries.
Beyond consequences primarily endured by the hapless veteran and his long-suffering family there remains the likelihood of a society having to deal with varying degrees of banditry.
Many of the famed pirates of the Caribbean were sailors discharged from the British Navy who, having become surplus to requirements following the Napoleonic Wars, found far more profitable employment for their skills of sailing, gunnery and hand-to-hand combat.
In more modern times - from Columbia to Mozambique - the members of Cold War armies backed by either the United States or former Soviet Union have created misery as bandits and guerrillas, long after their units were officially disbanded.
Troublesome former combatants have been a mainstay of criminal biker gangs too. Even since the conclusion of World War II in 1945, membership of these groups has been swelled by ex-servicemen following almost every war, from Vietnam to wars in the Balkans, Persian Gulf and Afghanistan . . .
If there is one conclusion to draw, it is that the devil makes work for idle hands. And all the more so if these hands have been trained to kill . . .
So when Trump gleefully speaks of waterboarding terrorists, he is not helping - just setting our world up for another 100 years of tit-for-tat violence.
To me, if Bush Jnr was a bumbling Claudius, this man is a rabid Nero. He must not become, as president of the United States, the most powerful man in the world.
Because, aside from his many other shortcomings, the methods Trump advocates have no real hope of defeating the so-called Islamic State.
It is a Hydra which will only thrive on more hate and violence.