This agreement made in 1925 in Switzerland was supposed to prevent a recurrence of a war between France and Germany especially. It was in hindsight obviously a total failure and helped cause World War II rather than prevent it. This is because it achieved German acceptance of its western borders with Belgium and France but said nothing about its recently changed borders on the east with Poland and Czechoslovakia. And World War II started because of German demands on Polish territory, i.e. Danzig.
For various reasons, Great Britain refused to support France's desire to guarantee the borders of Poland and Czechoslovakia and thus the appeasement of Germany and the unraveling of the terms of the Versailles Treaty started. And this was before the rise of Hitler and the Nazis. France made token agreements with Poland and Czechoslovakia but her agreement in Locarno prevented it from attacking Germany across the Rhine, which would be the only way possible to come to the aid of her eastern allies. Therefore her strategy of surrounding Germany with a strong alliance was reduced to nonsense.
The British diplomats in particular showed incredible stupidity in losing sight of the destabilizing and and aggressive potential of Germany. Because of fears of pushing Germany into the arms of the Soviet Union and of not appreciating France as a bulwark against a hostile Germany, the British virtually invited future German expansionism in the east. This was only 6 years after they helped create these boundaries in the Treaty of Versailles. In the new agreement Germany voluntarily agreed to the de-militarization of the Rhineland. Of course when Hitler marched his soldiers back into that area in 1936, scrapping the Treaty of Locarno, the British and French did nothing. So the Allied betrayal of Poland and Czechoslovakia in that treaty was for nothing.