On September 29, 1938, Neville Chamberlain, Edouard Daladier, Adolf Hitler and Benito Mussolini met at the Fuhrerbau in Munich and signed an agreement after midnight, in the early hours of September 30, that gave Germany the Sudetenland. The Czechoslovak delegation was in the building and was not called in; they were shown the result and told there was nothing to discuss. The Soviets were not invited.
The Sudetenland was the border region of Czechoslovakia with about three million ethnic Germans, a good part of the country's fortifications, and a large share of its industry. Hitler had been demanding it since the spring, and the Sudeten German Party under Konrad Henlein had been instructed to keep asking for what could not be granted. Czechoslovakia had mobilised on September 23, and its army was prepared to fight.
Chamberlain had flown to Berchtesgaden on September 15 and to Bad Godesberg on September 22, and each time Hitler had raised the demand. At Godesberg he presented a memorandum requiring immediate occupation by October 1 and a plebiscite in the disputed districts, and Chamberlain said it was an ultimatum and went home. The British cabinet rejected it, the fleet was mobilised, and gas masks were distributed.
What saved the negotiation was Mussolini, who proposed a compromise on September 28 that was, in substance, what Hitler wanted. Chamberlain flew out again. The agreement provided for German occupation in five stages between October 1 and 10, with an international commission to draw the boundaries and hold plebiscites, and a guarantee of the rest of Czechoslovakia that was never enforced.
The reasoning is still argued. Britain's Chiefs of Staff had advised that the country could not defend itself in 1938, the Dominions would not fight for the Sudetenland, and Chamberlain believed Hitler could be managed and that a European war would destroy everything. He came back to London with the piece of paper and said he had brought peace for our time.
What the agreement did was make the next step possible. Germany took the rest of Bohemia and Moravia on March 15, 1939, in breach of the agreement, which ended the policy. Britain and France guaranteed Poland on March 31 and declared war on September 3. When Hitler attacked the Soviet Union in 1941, Czechoslovak squadrons flew in the RAF and Czechoslovak units fought on the Eastern Front.