How the country was put together
The map people recognize is a nineteenth-century and early twentieth-century arrangement.
The Romanian state was not assembled by a single battle and a proclamation. It was assembled in stages, by elections, treaties, a war, and the collapse of empires that had sat on these lands for centuries. If you remember only one year, you will get the story wrong. 1859 and 1918 are the two hinges. They are not the same event.
By the middle of the nineteenth century, Wallachia and Moldavia were still separate principalities, under Ottoman suzerainty and watched closely by Russia and by the other European powers. A generation of boyars and younger men who had studied in Paris wanted a constitution, a shared prince, and an end to being treated as a passageway. In 1859, after arrangements made by the powers at the end of the Crimean War, the two assemblies elected the same man, Alexandru Ioan Cuza. That double election was the personal union. It was clever, and it was fragile. Cuza then had to make one administration out of two, reduce the legal power of the great landowners, and survive the people he had disappointed. He was forced out in 1866. A foreign prince, Carol of Hohenzollern, was brought in to make the new state look permanent to the courts of Europe.
Independence, in the full legal sense, came later, with the war of 1877–1878 against the Ottomans and the treaties that followed. Romania left that settlement with a recognized kingdom in all but name, and then a kingdom in name from 1881. It also left with a southern Dobruja question and with Constanța as a port that now faced the Black Sea as a Romanian city. None of that included Transylvania. Transylvania remained inside Hungary, and after 1867 inside the Hungarian half of the Habsburg monarchy. Romanian speakers there had parties, schools fights, and a memorandum. They did not have the state.
The First World War broke the frame. Romania entered late, was occupied, and then found itself on the winning side as Austria-Hungary came apart. In 1918, assemblies in Bessarabia, Bukovina, and Transylvania voted union with the kingdom. The diplomats spent the next years arguing the edges. The enlarged country is what people later called Greater Romania. It included many people who were not Romanian: Hungarians in Transylvania, Germans, Jews, Ukrainians, and others. A union voted in a crisis is not the same thing as a settlement everyone accepted.
The useful habit is to keep the stages separate. 1859 made one principality out of two. 1878 made that principality a recognized state with a sea. 1918 attached the lands inside the mountains and to the east, in the wreck of empires, and created a country whose map looked, for a while, like the widest claim the language could support. Almost every fight in the next hundred years, at home and with neighbors, is about what that map included and who was a full citizen inside it.