'Südost' was the German seizure of the rump of Czechoslovakia (15 March 1939).

Following the Munich agreement of September 1938, which gave Germany the Sudeten northern, western and southern border regions of Czechoslovakia, in November 1938 Emil Hácha succeeded Edvard Benes as president of the renamed Czecho-Slovakia, a federated republic comprising Bohemia and Moravia, Slovakia, and Carpatho-Ukraine. Stripped of its defensive frontier regions and thus deprived of its costly system of border fortifications, the new state was militarily indefensible.

In January 1939, negotiations between Germany and Poland broke down and Adolf Hitler, intent on war against Poland, needed to remove Czecho-Slovakia from the central European equation. On 13 January Hitler therefore scheduled a German invasion of Bohemia and Moravia for the morning of 15 March and, meanwhile, negotiated with the Slovak People’s Party and Hungary to prepare the dismemberment of the republic before the invasion. On 13 March, Hitler invited Tiso to Berlin, and on the following day the Slovak Diet met and unanimously declared Slovak independence. Carpatho-Ukraine also declared independence but Hungarian troops occupied it on 15 March and eastern Slovakia on 23 March.

Hitler summoned Hácha to Berlin and during the early hours of 15 March informed him of the imminent German invasion. Threatening a major air attack on Prague, Hitler persuaded Hácha to order the capitulation of the Czech army. Hácha suffered a heart attack during the meeting, and had to be kept awake by medical staff, eventually giving in and accepting Hitler’s surrender terms. Then on the morning of 15 March German troops entered Bohemia and Moravia, meeting no resistance. The Hungarian invasion of Carpatho-Ukraine did encounter resistance, but this was quickly crushed.

On 16 March, Hitler went to Czechoslovakia and from Prague Castle proclaimed Bohemia and Moravia to be the Reichsprotektorat Böhmen und Mähren (Protectorate of Bohemia and Moravia). Much of Slovakia and all of Subcarpathian Ruthenia were annexed by Hungary, while Poland occupied Zaolzie, an area with Polish minority, in October 1938.

The long-term German plan was to eliminate the concept of Czech nationality through assimilation, deportation and extermination of the intelligentsia, and in fact the intellectual elite and middle class represented a considerable proportion of the 200,000 people who passed through concentration camps and the 250,000 who died during German occupation. The 'Generalplan Ost' was created on the assumption that some 50% of the Czech population would be fit for 'Germanisation', and that the intellectual elite was to be removed not only from Czech territories but from Europe completely. The 'Generalplan Ost' was based on the assumption that it would be best if the intellectual elite emigrated overseas as even in Siberia it could become a threat to German rule.

The deportation of Jews to concentration camps was soon being planned and implemented, and the fortress town of Terezín was made into a ghetto way-station for Jewish families. On 4 June 1942 the Reichsprotektor, SS-Obergruppenführer Reinhard Heydrich, died after being wounded in 'Anthropoid', and his successor, SS-Oberstgruppenführer Kurt Daluege, ordered mass arrests and executions as well as the destruction of the villages of Lidice and Lezáky. In 1943 the German war effort was accelerated, and under Karl Hermann Frank, German minister of state for the Reichsprotektorat Böhmen und Mähren, some 350,000 Czech labourers were dispatched to the Reich. Within the protectorate, all non-war-related industry was prohibited.

For the population of the Reichsprotektorat Böhmen und Mähren, the German occupation was a period of brutal oppression. Czech losses resulting from political persecution and deaths in concentration camps reached a figure between 36,000 and 55,000. The Jewish population of Bohemia and Moravia, which had been 118,000 according to the 1930 census, was effectively eliminated: many Jews emigrated after 1939, more than 70,000 were killed, and 8,000 survived at Terezín. Several thousand Jews managed to live in freedom or in hiding throughout the occupation.

Even so, despite the estimated 136,000 deaths at the hands of the Nazi regime, the population in the Reichsprotektorat Böhmen und Mähren saw a net increase of about 250,000 in the period from 1939 to 1945.