BY all accounts the last summer in Britain before the outbreak of "the Great War" was preternaturally sunny.

If there was a cloud on the horizon it did not seem so threatening, merely the harbinger of a refreshing shower. It was an age often described as innocent, when pleasures were simple - "Stands the Church clock at 10 to three?/And is there honey still for tea?" - and society was stable.

Though it was a decade and more since the death of Queen Victoria, Britain was still a country she would have recognised without any trouble. Among the upper classes life was especially sweet. Girls still "came out" and spent months courting suitors and boys from certain backgrounds could count on allowances from their parents if they were disinclined to seek gainful employment. As devotees of Downton Abbey will attest, everyone knew his place and was kept firmly in it. Anyone interested in moving between stairs was deemed a misfit or an upstart.

Moreover, Britain before the war that was to end all wars was a country where government interference was negligible. As the historian AJP Taylor acknowledged: "Until August 1914 a sensible, law-abiding Englishman could pass through life and hardly notice the existence of the state, beyond the post office and the policeman." Putting aside Taylor's chauvinism, there is no doubting the truth of his statement. Taxes were small and regulations few. You could live where you liked and as you liked. No-one had a national insurance number and there was no need for a passport. Were you sufficiently well off you could do the Grand Tour without showing a document at a border. On top of all of which, there was no imperative to perform military service, unlike the citizens of other European countries. As Taylor wrote: "An Englishman could enlist, if he chose, in the regular army, the navy, or the territorials. He could also ignore, if he chose, the demands of national defence."

The war changed all of this. In four hellish years, Britain was dragged battered and bloodied into the 20th century. Some five million men marched away, one million of whom did not march back. Of those who did many were wounded physically and psychologically. They limped home to a place changed almost beyond recognition. Now, noted Taylor, "Their lives were shaped by orders from above; they were required to serve the state instead of pursing exclusively their own affairs." "Freedom of movement" was no more. News was "fettered"; licensing hours reduced. Even street lighting was limited. "The state established a hold over its citizens which, though relaxed in peacetime, was never to be removed and which the Second World War was again to increase."

Then as now people craved peace, but how to achieve it? It lies high on the wish list of everyone from Pope Francis to a wannabe Miss World, never more so than at Christmas, the season when "peace on earth" is on everyone's lips and "goodwill to all men" is sworn routinely. That festive desire for the fighting to stop was at its height during Christmas 1914, which was marked by a series of unofficial truces along the Western Front. On December 24 and 25, the guns fell silent, British and German soldiers exchanged seasonal greetings and sang carols. Some ventured into no man's land to exchange small gifts of food or cigarettes and, most famously, to play an impromptu game of football. Very soon, of course, the fighting recommenced.

Peace on earth: it seems such an easily achievable aim. All people have to do is stop trying to harm or kill other people. Once they do, then peace and harmony and contentment will follow. "All we are saying," sang John and Yoko, "is give peace a chance." And while many laughed at the two hippies who couldn't - or wouldn't - get out of bed, countless of us sang along. That was back in the days of the Vietnam War, which you didn't need to be an alumni of Sandhurst or West Point to realise was one of those wars that would not end decisively. Like most wars, it was messy, unnecessary and cruel. People died for no other reason than politicians decreed that they should.

Churchill may have declared that "to jaw-jaw is always better than to war-war", but time and again over the past century bullets have been fired and bombs dropped while the jaw-jawers argue over who should sit where and what the terms of a conference should be.

Protocol, it often seemed, was more important than matters of life and death. In the games the great and small powers play with the lives of their peoples there is no referee, no-one to step between two sluggers determined to pummel each other to pulp.

It is not, though, for the want of trying. In an essay written in 1960 titled Unity, EB White, author among other gems of the children's classic Charlotte's Web, recalled that in the year he was born, 1899, there was a peace conference held at the Hague. White couldn't recall what its outcome was. In fact, the participating nations agreed on various things, including the prohibition during war of "asphyxiating" gases, "expanding" bullets and the discharge of "projectiles or explosives from balloons". Most importantly, they also agreed on the creation of the Permanent Court of Arbitration to help settle international disputes.

Some six decades on, however, White had reasons plenty to be cynical, not the least of which were the two world wars which fell during his lifetime. During that period the world had become a much more dangerous and fragile place. Put simply, human beings were becoming ever more adept at killing one another. In that regard, the atomic bombings released on Hiroshima and Nagasaki in 1945 were seismic "advances", taking our ability to self-destruct to a new and quite spectacular level. White wanted a ban on the testing of all new such weapons but it never happened despite the compelling logic. For as he conceded, no sooner are agreements signed than a thousand selfish reasons arise for wanting to violate them.

Despair is the natural response to such a routine impasse. But if human beings are marked from other species by their capacity to kill one another on an industrial scale, they are also distinguished by their ability to find hope and optimism when common sense suggests there must be none.

"Most people," wrote White, "think of peace as a state of Nothing Bad Happening, or Nothing Much Happening. Yet if peace is to overtake us and make us the gift of serenity and wellbeing, it will have to be the state of Something Good Happening.

What is this good thing? I think it is the evolution of community, community slowly and surely invested with the robes of government by the consent of the governed. We cannot conceivably achieve a peaceful life merely by relaxing the tensions of sovereign nations; there is an unending supply of them. We may gain a breather by relaxing a tension here and there, but I think it is a fallacy that a mere easement, or diplomacy triumphant, can ever be the whole base for peace. You could relax every last tension tonight and wake tomorrow morning with the makings of war, all the familiar promise of trouble."

As White intimated, the reasons why peace is broken are myriad. One reads books on the "causes" of conflicts, such the two world wars, and one is struck by their Hydra-headedness. Of course, belligerent, inflexible, despotic, crazy individuals play their part but so too do circumstances. What would Hitler have achieved, for example, without rocketing inflation in Germany in the 1920s? In times of trouble people panic and look to their leaders, be they well-meaning or ill, for solutions. They don't take lessons from history or consider context. What concerns them is their immediate situation.

Hence the present rise in Europe of right-wing groups profiting from spiralling unemployment among disaffected young people with energy to burn. Many of them, one suspects, would rather have a job and a secure income than peace. It does not suit human beings to do nothing. We need goals, purpose, a future, and the more time spent without these the more idleness begins to grate and fester into something ugly and poisonous. We look then for reasons to explain our predicament. We rage against politicians and when they don't respond as we would like we turn on others, as the Nazis did on the Jews and the Hutus did on the Tutsis. There's always someone else better off than ourselves, always someone else who seems to be beating the system, always someone else to blame.

Meanwhile Hague Conventions come and go. War criminals are tried and given life sentences. The United Nations meets, jaw-jawing as Churchill demanded, but more often than not, equivocating, failing to pass "resolutions" while tens of thousands die. But most wise commentators believe we are better off with the UN than without it. In his 1998 Reith Lectures the military historian John Keegan took the view that if war is "to be driven to and beyond the horizon of civilisation, it will be because the United Nations retains both the will to confront unlawful force with lawful force and because the governments that lend it lawful force continue to train, pay and equip men of honour to carry out their orders".

For Keegan, those professionally engaged in making wars and trying to bring a halt to them, are as necessary as the violence they use. To end violence, he argued, you often need to use violence and we must "not shrink" from it. While many people may find this abhorrent, it is surely a truism. Another is that weapons are increasingly easy to get hold of, either over the internet or in an American supermarket. On holiday recently in Italy I could have bought a Beretta for just over £100 from a shop in a fashionable drag. Nor is there any likelihood of their owners abandoning them. The right to bear arms, the National Rifle Association insists, is as fundamental as access to a ballot box. But there's no doubt that modern guns are capable of killing many more people than even the repeating rifles used in the Wild West. So what can be done about them if they can't be taken away?

Keegan's solution, which appears to have fallen on deaf ears, is the restriction of their distribution and their means of production. "It is not true that the trade in cheap arms is a private commercial enterprise," he concluded. "Most cheap weapons have been released into the market by governments, often for political rather than commercial reasons." Some of these governments are trying to raise much-needed cash while others produce cheap weapons as they do plastic toys, electronic goods and clothes. It's about supply and demand. All of which and more contributed to what the Glasgow-born historian Niall Ferguson calls "the age of hatred", making the 20th century "by far the bloodiest in all human history."

It is interesting to note, however, that while the century past was stained by "extraordinary violence", by far the worst of this happened in its first 50 years, which was due, of course, to the carnage of the two world wars. Ferguson's aim in The War Of The World: History's Age Of Hatred was to explain how this came about. In so doing he took issue with Tolstoy who, in War And Peace, attributed responsibility for wars to a few bad and mad men. "Megalomaniacs may order men to invade Russia," wrote Ferguson, doubtless thinking of Napoleon and Hitler, "but why do men obey?"

It is a huge question and one not even Ferguson could satisfactorily resolve. Ultimately, he reckoned, there were three things which explain why the 20th century erupted in an orgy of killing: ethnic conflict, economic volatility and empires in decline. "We shall avoid another century of conflict only if we understand the forces that caused the last one," wrote Ferguson, "the dark forces that conjure up ethnic conflict and imperial rivalry out of economic crisis, and in doing so negate our common humanity. They are forces that stir within us still."

Some may see this as an essentially pessimistic assessment. Others, myself included, are struck by its validity. Historians, we are often reminded, are custodians of the past, being uncomfortable in the present and rightly wary of predicting the future. For while we are very different from our distant and near forebears we are like them in many respects too, sharing the same animal tendencies.

But to those who say we are condemned to repeat the mistakes of the past there is cheer to be found in evolution, in humankind's wherewithal to invent solutions to problems which threaten its existence, be it the melting of the ice cap or a dearth of drinking water. Slowly but surely we are becoming more informed, better educated and increasingly sophisticated, and with that we are able to understand how other people live and the effect our actions and behaviour have on them. Thus, instead of getting progressively worse human beings are, in fact, becoming more civilised and less violent.

That at any rate is the thesis of the Canadian psychologist Steven Pinker, which he expounded earlier this year during his Gifford lecture at Edinburgh University. Our forebears, he said, were different from us, not least in respect of peace. Tribal warfare was nine times as deadly as war and genocide in 20th century Europe. The murder rate of medieval Europe was more than 30 times what it is today. Wars between developed countries have virtually ended, though of course they rage still in many places, from Afghanistan to Syria and the Central African Republic. But even in the developing world, wars kill a fraction of the people than they did a few decades ago. Rape, hate crimes, domestic violence, deadly riots, child abuse, cruelty: all are substantially down.

Thus there is hope. Savour that thought as you tuck into your turkey and stuffing.