The Principality of Moldavia and the Principality of Wallachia were two separate political entities that existed in the region now known as Romania. In 1859, Alexandru Ioan Cuza was elected as the ruler of both Moldavia and Wallachia, creating a de facto union known as the United Principalities of Moldavia and Wallachia.
Transylvania, on the other hand, was part of the Austro-Hungarian Empire at that time. It wasn't until the end of World War I in 1918 that Transylvania became part of Romania.
The unification of Transylvania with Moldavia and Wallachia to form the modern state of Romania was formally proclaimed on December 1, 1918. This date is celebrated in Romania as the National Unity Day.
Therefore, the unification of Transylvania and Moldavia to make Romania happened in 1918, not through a direct union between Transylvania and Moldavia, but through the union of Transylvania with the already united Moldavia and Wallachia.
