The Locarno Pact, actually a group of seven treaties (hence its other name: the Locarno Treaties), was signed on 1 December 1925 with the aim that peace continued in Europe despite the German government's disapproval of the Treaty of Versailles, which formally concluded the First World War (1914 to 18). The pact is named after the Swiss town of Locarno, where the delegates from seven European nations met.

The main points covered by the Locarno Pact included treaties of mutual guarantee to protect existing borders in Western Europe and a promise both not to use war as a tool of foreign policy and to resolve disputes by diplomacy. Significantly, the treaties did not resolve the issue of Germany's eastern borders, particularly with Poland. The Locarno Pact did ensure peace for 11 years, but it was irreversibly broken when Adolf Hitler, the leader of Nazi Germany, invaded the demilitarised Rhineland in 1936, a breach that was followed by a series of other more serious acts of aggression, which eventually led to the Second World War in 1939.

Germany lost WWI, and the Paris Peace Conference, which drew up the Treaty of Versailles in June 1919, decided its new borders. The treaty had also stated that Germany was to blame for the war, must make reparations to the victors, and its armed forces would be restricted both in numbers and in the types of weapons it could have. There had been wide public outrage in Germany at the terms of the Versailles treaty, but by the 1920s, this had died down, and the only simmering point of real discontent was Germany's redrawn borders.

Germany had been obliged to give back to France the regions of Alsace and Lorraine, it lost control of coal-rich Saar region and Danzig (Gdańsk), and the Rhineland was demilitarised. Belgium received Eupen-Malmedy, and Denmark gained Northern Schleswig. Germany lost all of its colonies. The issues of most contention from a German point of view were Poland's gain of the industrial region of Upper Silesia and a corridor of territory that led to the Baltic coast (the 'Polish Corridor'). This corridor cut off East Prussia from the rest of Germany. In total, the Allies had shrunk German territory by around 13% compared to its pre-war state.

To settle these issues, several delegations of European states met in the Swiss lakeside town of Locarno in 1925. Germany no longer had a monarchy (ex-Kaiser Wilhelm II had shouldered much of the blame for WWI as an inveterate warmonger) but was then a constitutional republic, its economy was recovering, and it was paying its war reparations. Now, it seemed to WWI's victors, was the time to show a little leniency over what many regarded as the rather too harsh Treaty of Versailles.

The delegations at the Locarno Conference, who were meeting under the auspices of the League of Nations, came from Britain, France, Italy, Germany, Belgium, Poland, and Czechoslovakia. The conference was held at the Palazzo del Pretorio from 5 to 16 October. Aristide Briand (1862 to 1932), the French statesman who had been prime minister during WWI (and who would be again after the conference), led the French delegation. The British were led by the Foreign Secretary Austen Chamberlain (1863 to 1937), and the Germans by their foreign minister and former chancellor, Gustav Stresemann (1878 to 1929). Benito Mussolini (1883 to 1945), the leader of Fascist Italy, also attended, as did future British prime ministers Neville Chamberlain and Winston Churchill.

Briand, above all, wanted Germany to promise it would continue to respect its new frontiers. Germany agreed only that it would not use force to change its western borders. Regarding its eastern borders, the issue was less clear, and Germany merely agreed to the use of arbitration with (only) the governments of Poland and Czechoslovakia. As the historian A. J. P. Taylor put it, the frontier with Poland "might be tolerated; it could not be confirmed" (82). To strengthen Poland's position, France reaffirmed its alliance with that state, and it did the same with its alliance with Czechoslovakia, the trio effectively promising to come to the aid of any other if attacked. It was also agreed that if France acted to protect the frontiers of either state, this could not be interpreted as an act of aggression against Germany.

France, always concerned first and foremost that Germany might attack it again, was relieved by Germany's pacific overtures in a western direction and was glad to keep both Alsace and Lorraine and maintain the Rhineland as a demilitarised zone. Britain and Italy both guaranteed this position. Germany was happy that Cologne was finally relieved of its military occupation, that Allied inspections of its military would end, and that the Allied troops would leave the Rhineland by 1930 (not, as previously agreed at Versailles, by 1935).

Belgium, Germany, and France all agreed never to attack each other except "in legitimate self-defence" (an ambiguous term open to multiple interpretations). In addition, Britain agreed in principle to help defend France if that state were ever attacked. The guarantee was meant to be temporary, but as no alternative was agreed upon later, "Great Britain was committed – for the first time in her history – to peacetime alliance with a continental Great Power" (Taylor, 148). The commitment, everyone hoped, was merely one that need only be written down rather than ever be executed. The British regarded their position as less of an outright promise to protect France and more of the establishment of a moral duty to do so, a subtle difference which allowed for non-action at the British government's discretion. Belgium was given a similar guarantee; Poland and Czechoslovakia were not.

Everyone at the Locarno Conference promised to resolve future disputes only by "pacific means". If diplomacy between the direct parties involved failed, then disputes were to be presented for arbitration before a specially formed commission or such international bodies as the Permanent Court of International Justice. There was the flaw that Germany had here insisted that the arbitration provision did not apply to disputes between Germany, Czechoslovakia, and Poland, which predated the pact. Finally, the desire was clearly expressed to henceforth make some attempt at collective disarmament.

Everyone seemed reasonably content with the treaty agreements. France had maintained the status quo on its border with Germany and gained some sort of promise of protection from Britain. The British were content that stability in Europe could continue, and it had not overcommitted itself to any future action either with or against another state. Germany was content that it was no longer being treated as an international pariah and that it had gained some freedom of diplomatic movement regarding its eastern borders. Mussolini, really only invited to make Britain seem more impartial than it was, was flattered to be sitting at the same table as the great democratic powers, despite being a despot. Nobody seemed to miss Russia or the United States in these discussions to maintain peace.

The seven treaties were approved on 16 October in Locarno and then formally signed in the Gold Room of the British Foreign Office in London on 1 December that same year. Leaders told their respective populations that their sojourn in Locarno had been a success and that a lasting world peace was now assured.

Chamberlain, Briand, and Stresemann were awarded the Nobel Peace Prize (Chamberlain in 1925 and the others the next year) for their work with the Locarno Pact. A lasting peace in Europe did now seem perfectly possible if everyone was willing to keep talking to each other. This was especially so when Germany was invited to finally join the League of Nations in 1926. The idea of cooperation and discussion in international affairs, which the pact fostered, became known as the 'spirit of Locarno', and it contributed to additional treaties of cooperation such as the Kellogg-Briand Pact of 1928, signed by 63 countries, which formally made waging war an illegal act.

The Locarno Pact did have the unfortunate consequence that future leaders, less inclined to peace, would regard the agreements of 1925 from a different perspective and interpret them quite differently from those who had signed them. There was already in 1926 the differing perception that, for some, Locarno was the end of negotiations, while for others, it was only the beginning. This meant that future diplomatic clashes became inevitable. While France thought it could now avoid making any future concessions, Germany thought Locarno would be the start of more concessions.

There remained, too, the serious problem of who would really enforce breaches of the pact, a pertinent question when Britain was reluctant to involve itself outside of its already crumbling empire, and the United States insisted on an isolationist position. It was one thing to promise aid on paper and quite another for a state to send troops across a continent. Aggressor states were all too aware of this reality.

Another unfortunate consequence of the pact was that the agreements for mutual assistance made Britain, France, and Italy rather complacent. Thinking that such an agreement was a sufficient deterrent against any would-be aggressor, none of the three actually set about establishing some sort of military cooperation in reality. When acts of aggression came along and broke the illusion of a peaceful international community, there was a distinct absence of any unified response or combined retaliatory mobilisation of troops.

The Locarno Pact was finally ripped to shreds when Adolf Hitler (1889 to 1945) sent troops into the demilitarised Rhineland in 1936 and then proceeded to occupy other neighbouring states. Hitler had actually reaffirmed the territorial decisions of the Locarno Pact in 1935, but this was merely part of his strategy to confuse rival leaders as to what he was really up to, and he openly repudiated the pact the very next year. The policy of appeasement towards Hitler pursued by Britain and France, a policy encouraged by the sentiment of the Locarno Pact, was finally ended by the invasion of Poland in 1939, which started the Second World War (1939 to 45).

The Locarno Pact had, perhaps, been a pivotal moment in the history of the first half of the 20th century: "Its signature ended the first World war; its repudiation eleven years later marked the prelude to the second" (Taylor, 82). In short, a reliance on mere legal jargon, an unrealistic belief that peace was attractive to all leaders, and an absence of concrete promises of assistance based on practical military preparations all meant any aggressive state could and did act with impunity through the 1930s.