Below is a picture of the entrance of Michael the Brave into Alba Iulia, Transylvania in 1599. At this point, the three principalities of Moldavia, Wallchia and Transylvania were not united into the country called Romania. During the period of a year, under his rule they were united. After the death of Michael, Transylvania slowly became a part of the Kingdom of Hungary...
This is a picture of the delegations of 1918, when they met at Alba Iulia. This was where the many peoples of Transylvania gathered to decide what country they wanted to be a part of. At this time, Hungary and Austria were in the process of breaking apart and the new nations would be born around them. Transylvania had a large population of Romanians and the German Saxons and they voted to unite with Romania.
As it stood, it didn't matter because for the first time since 1599-1600 they would become a part of Romania. Romania was in fact born after the retreat of the Ottoman Empire in the 1800s. Alexander John Cuza was the first Prince of Romania in 1859. In 1866 Carol I of Romania (which at that point only was Wallachia and Moldavia,) became Prince of Romania. He became King of Romania in 1881-- Transylvania was still not a part of Romania at this time.
It would join after 1919 and the Treaty of Paris.