100 years ago, Europe was in the middle of the First World War. In many ways, not much had changed since the 4th Century BC when Alexander the Great was fighting the Persians. Whereas in that war you had two main Empires fighting each other for a greater share of the pie, in the 1910s, you had opposing Empires fighting each other for a greater share of the pie. Wars were in no way a one off in the thousands of years before the 20th century, and even when they weren't happening, war was more often than not, in the air, with different powers most often viewing each other as rivals to be contained. Today, the situation is totally different; the nation states of Europe now see each other as neighbors to cooperate with and not rivals to be contained. Yes, arguments between neighboring countries do happen but they are no comparison in any sense; whenever, say, France and Britain have had a disagreement, it has been over border control at Calais and the Common Agricultural Policy; not over the Channel Islands or colonies in Africa.
That, I believe, is at the heart of it all. Conflicts over territory are far less common in the world today than they used to be; territory is no longer something that you simply acquire, and with that, territorial expansionism, a leading cause of war, is (almost) unheard of. But just why have our attitudes territory changed so much? First of all, we live in a Europe, and a wider world, where the vast majority of countries are nation-states, and in Europe, for the most part, international borders reflect ethnic and linguistic countries. Therefore, it doesn't make sense to go round annexing territory where the population is of a different nationality. The belief in territorial self-determination has been critical, stating that the population of any given area should have the final say, meaning that traditional ways of acquiring territory, such as through marriage or conquest, are now seen as illegitimate. This change, along with the related belief that all men are created equal, has replaced expansionist practices such as colonialism and Lebensraum. Lebensraum, for example, was a German belief, that they, the Germans, were superior to the peoples of Poland, Russia and other Slavic countries, and that they therefore had to right to conquer and enslave them and steal their land. It was perhaps the most important cause of WW2, and therefore its demise as an ideology, and of any similar expansionist ideologies, has made any kind of repeat of WW2 completely out of the question. The end of colonialism, in which certain European countries had large Empires on other continents, has meant that nations such as Britain and France have one less thing to fight over. It is no coincidence then, that the demise of Empire and this long lasting period of Peace have come about at the same time. Then of course, there is Constitutional Democracy and the fact that under such a system, governments can no longer go to war on a whim and expect their populations to unquestionably serve up. The idea that Democracy brings peace is as old as Thomas Paine. It can only be a good thing, then, that Constitutional Democracy is the dominant political system across Europe and the wider industrialized world today. But in addition to the important political changes that have just been described, I also remember someone arguing on youtube that the post-war economic miracles that took place in Europe, South Korea and Japan, helped to prove once and for all that territorial expansion was not a prerequisite for economic expansion. Today, when countries do compete, it is to attract foreign direct investment, to get to the top of the PISA rankings, eetc, and to out-perform each other in ways that do not involve any kind of territorial aggrandizement.
The three main Europe-centered conflicts of the Twentieth Century, the First World War, the Second World War and the Cold War, did a lot to bring about the conditions described above. Not only did the first two show just how destructive war can be, and thus act as a deterrent against future wars, a notable achievement of the Cold War was that it served to further consolidate unity between nations which were on the same side. In the West, the capitalist nations realized that they had to pull together and become allies in the face of the communist threat while in the East, the Red Army made sure that no conflicts broke out between the Warsaw Pact countries. Thus, when the Iron curtain finally came down, the allied countries in the West, for example, were now so used to being allied with each other, that they weren't going to suddenly start fighting each other again, while more importantly, the factors described in the previous paragraph were now firmly established.
Thus, while I was a staunch remainer in the lead up to the referendum, and count myself as a pro-European even now (though I do accept the result of the referendum), I feel that it was neither wise nor correct for the Remain Campaign to go so far as to say that the EU was what had prevented war breaking out. Such an argument only allowed Leave supporting politicians to get another chance at ridiculing the Remain Campaign, a chance which individuals such as Nigel Farage and Daniel Hannan most definitely took.