The national flag of the former Kingdom of Yugoslavia was blue-white-red in the horizontal sense against a vertical staff. The common national civil flag was the same as historic Pan-Slavic flag approved at the Pan-Slavic Congress in Prague, 1848.
The naval ensign (war flag) of the Kingdom of Yugoslavia is blue-white-red with the simplified lesser coat of arms: On one third of the ensign length there shall be the state coat of arms with the crown. The height of the arms and crown (without the globe and cross) shall be half of the ensign height.
The flags of the Kingdom were in official use from 1922 until the Kingdom of Yugoslavia was occupied by Axis powers in 1941. After that, the flag was used by the officially recognized government in exile, diplomatic representatives, and the Allies until 1945. During the Second World War, Yugoslav Army in the Fatherland (also known as Chetniks) continued to use the flag.
The Kingdom of Serbs, Croats and Slovenes was established on December 1, 1918 and was renamed the Kingdom of Yugoslavia on October 3, 1929. The state's first flag was officially adopted in 1922. All Yugoslav flags (including the first ones) were variations on the Pan-Slavic flag adopted at the Pan-Slavic Congress in Prague in 1848. The Pan-Slavic flag was a plain blue-white-red tricolor in the horizontal sense against a vertical staff, and the national flag and civil and state ensign during the 1918-1943 period (Kingdom of Yugoslavia) was exactly the same. The naval ensign during the period was the blue-white-red tricolor with the simplified lesser coat of arms of Yugoslavia.