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The national flag of the Czech Republic (Czech: státní vlajka České republiky) or flag of Czechia (vlajka Česka), or Czech Flag (česká vlajka) is the same as the flag of the former Czechoslovakia. Upon the dissolution of Czechoslovakia in 1993, the Czech Republic kept the Czechoslovak flag while Slovakia adopted its own flag. The first flag of Czechoslovakia was based on the flag of Bohemia and was white over red. This was almost identical to the flag of Poland (only the proportion was different), so a blue triangle was added at the hoist in 1920. The flag was banned by the Nazis in 1939 as they established a government nominally in control of Bohemia and Moravia, and a horizontal tricolour of white, red, and blue was used for the duration of the war. The 1920 flag was restored in 1945.

Source: Wikipedia