In 1918, when the Austro-Hungarian Empire collapsed and the Second Reich lost WWI due to internal reasons, the Allied countries decided to transfer the originally German Sudetenland to a new country - Czechoslovakia.
A few comments on the so-called Czechoslovakia. It was an artificial state formed by the will of the Entente (not just one - there was also Yugoslavia). No wishes of the peoples were taken into account. The Czechs, for example, lived quite peacefully and well in Austria-Hungary, it was a prosperous region. Slovakia is worse, especially considering its location on the territory of the "Lands of the Crown of St. Stephen" and Hungarian policy, but their position was also quite good.
The Germans, who had lived in the Sudetenland for centuries, found themselves in this artificial state, and their position was quite unenviable. During the interwar period, German interests in the region were not really represented, and only after the National Socialist Revolution in neighboring Germany, in 1933, the Sudeten German Party emerged. The party initially fought for autonomy, but later, as the position of the German state grew stronger, the party leadership, headed by Konrad Heinlein, a German nationalist and National Socialist;
The fight for the reunification of the Sudetenland was one of the components of the political program of the National Socialists, aimed at the reunification of the German state. Similar to Austria, the AH enjoyed great authority and support among the population in this seized land. After the Anschluss and Heinlein's meeting with Hitler, the first (spring) Sudeten crisis took place. Internal provocations in the Sudetenland became more frequent, when the Sudeten German Party openly demanded reunification with Germany in April, and the Czechoslovak army carried out a partial mobilization on May 21. At that time, however, international pressure on Germany was unusually strong, and unification was postponed.
The summer of 1938 passed relatively calmly. The Wehrmacht continued the gradual renewal of its weapons, which is not, however, a sign of preparation for war. It is worth considering that Germany until 1935, by decision of the imposed Versailles Treaty, de facto had no army. The Reichswehr was more like a police force, limited in numbers and weapons, and deprived of the General Staff. Germany's economy continued to strengthen, and relations with Italy gradually improved.
The second Sudetenland crisis occurred in September. As then, the Czechoslovak army mobilized and began preparing for war, counting on the Allies to get involved in a global confrontation in order to "save" Czechoslovakia (and in fact, to preserve the artificially annexed territories with a German population). The Allies, however, felt unprepared for such a step, which is quite understandable. In 1938, the rearmament program in England and France was just beginning, and an early war could have ended in a dramatic defeat for the Allied powers, given the previous German rearmament.
Eventually, the Munich Agreement was signed, legally and internationally securing the entry of the Sudetenland into the Third Reich. Heinlein was given the post of head of the Reichsgau Sudetenland, and the Sudeten German Party merged with the NSDAP.
The loss of territory triggered an internal crisis in the rest of the artificial state. Within months, Czechoslovakia was losing territory to Poland and Hungary, and in March the political crisis escalated to the point that the Carpathian Ruthenians and Slovaks revolted. Apparently, the AH decided to intervene in the growing crisis, and secured the political sovereignty of the Slovak Republic, led by Jozef Tiso. The Slovak people took their independence quite positively. The Czech people gained national autonomy within the Reich - in the form of the Reich Protectorate of Bohemia and Moravia, which had its own president, currency, limited police and armed forces, and a separate ruling party. The Ruthenians, unfortunately, were occupied by Hungary, which did not take their national interests into account.
The Allies used the March collapse of the Czechoslovak artificial state as a pretext for rearmament, accusing the AH and the National Socialists of "inability to negotiate";
In fact, Hitler did not violate the Munich Agreement: Czechoslovakia was not annexed to Germany, and its parts received either autonomy or complete independence (except for Carpathian Ruthenia). Thus, accusations of "territorial annexation" of these territories were, at the very least, far-fetched and unfounded.