[5][6][7][8][9][10] Its territory, in addition to the traditional Transylvanian lands, also included the other major component called Partium, which was in some periods comparable in size with Transylvania proper.

In 1563, Giorgio Blandrata was appointed as court physician, and his radical religious ideas increasingly influenced both the young king John II and the Calvinist bishop Francis David,[21] eventually converting both to the Anti-Trinitarian (Unitarian) creed.

The Principality of Transylvania was established in 1570 when John II renounced his claim as King of Hungary in the Treaty of Speyer (ratified in 1571),[12][22] and became a Transylvanian prince.

[24] Upon the death of John II in 1571 the Royal House of Báthory came to power and ruled Transylvania as princes under the Ottomans, and briefly under Habsburg suzerainty, until 1602.

[21] which afforded religious liberty and political autonomy, the restoration of all confiscated estates, the repeal of all "unrighteous" judgments, and a complete retroactive amnesty for all Hungarians in Royal Hungary, as well as his own recognition as the independent sovereign prince of an enlarged Principality of Transylvania.

The Treaty of Vienna (1606) guaranteed the right of Transylvanians to elect their own independent princes, but Georg Keglević, who was the Commander-in-chief, General, Vice-Ban of Croatia, Slavonia and Dalmatia, was since 1602 Baron in Transylvania.

Gábor Bethlen, who reigned from 1613 to 1629, perpetually thwarted all efforts of the emperor to oppress or circumvent his subjects, and won reputation abroad by championing the Protestant cause.

[28][29] The Unio Trium Nationum (Latin for "Union of the Three Nations") was a pact of mutual aid codified in 1438 by three Estates of Transylvania: the (largely Hungarian) nobility, the Saxon (German) patrician class,[30] and the free military Székelys.

[33] The coalition of the "Three Nations" retained its legal representative monopoly under the prince as before the split of the medieval Hungarian Kingdom occasioned by the Ottoman invasions.

According to Dennis P. Hupchick, though there were occasional clashes between the Hungarian plainsmen and the Székely mountaineers, they were united under the patronymic "Magyars" and, with Saxon support, formed a common front against the predominantly Romanian peasantry.

Nicolaus Olahus, Primate of Hungary stated in the book Hungaria et Athila in 1536 that in Transylvania "Four nations of different origins live in it: Hungarians, Székelys, Saxons, and Vlachs"[34][35][36] Based on a work by Antun Vrančić (1504–1573), Expeditionis Solymani in Moldaviam et Transsylvaniam libri duo.

De situ Transsylvaniae, Moldaviae et Transalpinae liber tertius, more estimations exist as the original text is translated/interpreted in a different way, especially by Romanian and Hungarian scholars.

According to Hungarian interpretations, Vrančić wrote about the inhabitants of Transylvania and about the Romanians: "The country is inhabited by three nations, Székelys, Hungarians, and Saxons; I should also add the Romanians who – even though they easily equal any of the others in number – [37] have no freedom, no aristocracy, no right of their own, besides a small number living in the Haţeg district, where the capital of Decebalus is believed to have stood, and who, during the time of John Hunyadi, a native of those places, were granted aristocratic status because they had always taken part in the struggle against the Turks.

[41] Károly Kocsis and Eszter Kocsisné Hodosi argue that the Hungarians were the most numerous ethnic group before the second half of the 17th century, when they were exceeded by Romanians.

[48] According to Louis Roman, various works from the XVII century claim that Romanians were the most numerous ethnic group in Transylvania during that time, including those of Johannes Tröster, Grigore Ureche, and Miron Costin.

Evliya Çelebi, accompanying Ali Pasha's army into Transylvania in 1661, reported vast areas, comparable in size to counties, being reduced to ashes, entire villages being put to the sword, and groups of 3,000–8,000 captives.

The Transylvanian populations suffered huge losses, the Partium and the counties of Belső-Szolnok, Doboka, Kolozs, Közép-Szolnok, and Kraszna were laid waste.

[50] According to official estimates made by the Austrian administrative authority (Verwaltungsgericht) dating from 1712 to 1713, the ethnic distribution of the population in Transylvania is as follows: 47% Hungarians, 34% Romanians, 19%, Saxons.

[57] The Romanian peasantry, which flooded into Transylvania in this way, could take the place of the Hungarian, Székely and Saxon population decimated by the vicissitudes of the war, and their remaining real estate and property, without any difficulties.

[57] As a result of heavy taxes and hard services demanded, many serfs of the principalities of Moldavia and Wallachia migrated into Transylvania where the laws were more favorable.

[51] After the death of Prince Michael I Apafi, between 1690 and 1703, many people emigrated from Transylvania due to the tax burdens of the government that did not take into account the local circumstances.

This phenomenon, also existing in the Middle Ages during the foundation of Moldova and Wallachia,[58][59] was amplified after György Dózsa's rebellion of 1514, the religious persecutions and the worsening standard of living of Romanian Transylvanians.

The Cluj Diet of 1593 constated that "our haughty serfs, gathered in large groups and raised, immediately can go to the Romanian countries, even if they commit any illegality, they go there freely".

In 1602 Giorgio Basta ordered the dwellers of Bistrița to guard the roads, the passes and the paths towards Moldavia, to prevent the Transylvanian serfs getting the Principality.

[54] On other hand, according to Bolovan Ioan and Ștefan Meteș, the fact that Romanians belonged to the Orthodox Church and not to any Western Christian denomination was the cause of their remaining of political, economic and cultural inferiority to Hungarians, Szekelys and Saxons, making them more willing to emigrate towards Moldova and Wallachia.

It is also true that a small part of the Romanian nobility, as much as survived after the attempts of the Hungarian royalty to Catholicize in the previous centuries, in some places embraced one of the new reformed confessions.

[41] Food shortages, the famine of 1684–1686, caused by an increase in the price of grains, lead some of inhabitants to leave Transylvania, and many of the villages in the Fundus Regius remained abandoned.