Why did Britain, France, and the United States embrace the policy of appeasement?

Answer:

Policy of appeasement is the name by which the conciliatory policy carried out by Neville Chamberlain as prime minister of the United Kingdom, before the Second World War has been historically known.

Scared by the horrors of the First World War, as the biggest conflict known until 1914, not a few European politicians wanted to keep peace with the Germany of the Third Reich, regardless of the requests directed by the aggressive Nazi regime, which meant allow Hitler's constant infringements of the various international treaties, as happened with the militarization of the Rhineland, the western German region where the Treaty of Versailles in 1918 had prohibited Germany from establishing military forces, arsenals or fortifications. When Hitler sent in 1936 Wehrmacht troops to park in the Rhineland, Britain refused to protest this rupture of the Treaty of Versailles. Without British support, France also accepted without complaint any infringement of that treaty.  

The policy of appeasement prevented the same year of 1936 that Britain and France impose sanctions on Germany and Italy for their military intervention in the Spanish Civil War, against the agreements taken between these countries to not provide war support to the Spanish sides in conflict. Nor did they sanction the Soviet Union in its support for the Republican side in the Civil War.

A similar situation was experienced with the issue of German rearmament carried out by the Third Reich since 1933, despite the fact that the Treaty of Versailles established maximum troop limits for the Reichswehr (the German army of the Weimar Republic) and greatly reduced the scope of the German navy and military aviation. Neither France nor Britain protested against such an infraction, which allowed Hitler to organize much more powerful armed forces, the Wehrmacht.

The culminating moment of this policy was the 1938 Munich Conference, in which Chamberlain accepted the guarantees offered by Hitler to maintain European balance, leaving Czechoslovakia to German ambitions. However, on that occasion Neville Chamberlain seriously considered having avoided, and not only postponed, an armed conflict with Nazi Germany. In fact, after celebrating the Munich Accords, he returned by plane to Britain and when he got off the ship Chamberlain issued a famous statement to the press gathered at the airfield, noting that the Munich Accords were the "peace for our time", which won him applause from British public opinion that he believed he had really avoided a war.

The subsequent invasion of Poland in 1939, less than a year after the Munich Accords, ended up sinking the policy of appeasement, which led to the defeat of Chamberlain in a vote of censure in the House of Commons in May 1940, in front of Winston Churchill, who had been very hard in his criticisms of Chamberlain after his agreements with Hitler.