Before the said union took place, World War One intervened, and Montenegro entered it on the side of Serbia and the allies. When the Great War was over, Montenegro conjoined the newly created Kingdom of Serbs, Croats, and Slovenes and later the Kingdom of Yugoslavia.
It was the big precedent in history because it was the first time that South Slavs were united in a mutual state. However, at the very start, there were severe political and economical problems in the state. The territory of Montenegro became a part of Zeta banovina, and the name Montenegro was officially discarded.
This period was also marked by turmoil, with WWII coming very soon after the Great War, and where Montenegro was in a zone occupied by Italians. This period was marked by the Uprising in Montenegro, where three-quarters of the occupied territory was liberated. Soon after the Italian capitulation, Montenegro was occupied by Germans.
After WWII, the devastation was out of proportion, leaving millions dead, homeless, and displaced in every possible way. For the Balkans area, also including the Montenegro history course, it finally ended in 1944.
Soon after, the communist federation of Yugoslavia was established, a state where Tito was determined to create an environment where no ethnic group will dominate the political landscape. In this environment, Montenegro became one of the six republics, together with Serbia, Macedonia, Croatia, BiH, and Slovenia.
This use to be a tight union, but nothing lasts forever, and neither did Yugoslavia. The union became fragile after Tito’s death in 1980, and it led to the independence of former Yugoslavian republics.