The Locarno Treaties were seven agreements negotiated at Locarno, Switzerland on 5-16 October 1925 and formally signed in London on December 1, in which the World War I western European Allied powers and the new states of central and eastern Europe sought to secure the post-war territorial settlement, in return normalising relations with defeated Germany.
The Locarno discussions arose from exchanges of notes between Britain, France and Germany over the summer of 1925 following German foreign minister Gustav Stresemann's February 9 proposal for a reciprocal guarantee of his country's western frontiers as established under the unfavourable 1919 Treaty of Versailles, as a means of facilitating Germany's diplomatic rehabilitation among the western powers.
The principal treaty concluded at Locarno was that between Germany, France, Belgium, Britain, and Italy, under which the first three signatories undertook not to attack each other, with the latter two acting as guarantors. In the event of aggression by any of the first three states against another, all other parties to the treaty were to assist the country under attack.
Germany also signed arbitration conventions with France and Belgium and arbitration treaties with Poland and Czechoslovakia, undertaking to refer disputes to an arbitral tribunal or to the Permanent Court of International Justice.
France signed two further treaties with Poland and Czechoslovakia respectively, pledging mutual assistance in the event of conflict with Germany. These essentially reaffirmed existing treaties of alliance concluded by France with Poland on 19 February 1921 and with Czecholsovakia on 25 January 1924.
The Locarno Treaties were regarded as the keystone of the improved western European diplomatic climate of the period 1924-30, though tension persisted in eastern Europe. The "spirit of Locarno" was seen in Germany's September 1926 admission to the League of Nations, the international organisation established under the Versailles treaty to promote world peace and co-operation, and in the subsequent withdrawal (completed in June 1930) of Allied troops from Germany's western Rhineland.
One notable exception from the Locarno arrangements was, however, the Soviet Union, which saw western detente as potentially deepening its own political isolation in Europe, in particular by detaching Germany from her own understanding with Moscow under the April 1922 Treaty of Rapallo.
The Locarno spirit did not survive the revival of right-wing German nationalism from 1930. Proposals in 1934 for an "eastern Locarno" pact securing Germany's eastern frontiers foundered on German opposition and on Poland's insistence that her 1920 territorial gains from the Soviets should be covered by any western guarantee of her borders. Germany formally repudiated her Locarno undertakings in sending troops into the demilitarised Rhineland on 7 March 1936.