Alba Iulia in the context of "Marie of Romania"

Transylvania (Romanian: Transilvania [transilˈvani.a] or Ardeal; Hungarian: Erdély [ˈɛrdeːj]; German: Siebenbürgen [ˌziːbm̩ˈbʏrɡn̩] or Transsilvanien; Transylvanian Saxon: Siweberjen) is a historical and cultural region in Central Europe, encompassing central Romania. To the east and south its natural border are the Carpathian Mountains and to the west the Apuseni Mountains. Broader definitions of Transylvania also include the western and northwestern Romanian regions of Crișana and Maramureș, and occasionally Banat. Historical Transylvania also includes small parts of neighbouring Western Moldavia and even a small part of south-western neighbouring Bukovina to its north east (represented by Suceava County).

Transylvania is known for the scenery of its Carpathian landscape and its rich history, coupled with its multi-cultural character. It also contains Romania's second-largest city, Cluj-Napoca, and other very well preserved medieval iconic cities and towns such as Brașov, Sibiu, Târgu Mureș, Bistrița, Alba Iulia, Mediaș, and Sighișoara. It is also the home of some of Romania's UNESCO World Heritage Sites such as the Villages with fortified churches, the Historic Centre of Sighișoara, the Dacian Fortresses of the Orăștie Mountains and the Roșia Montană Mining Cultural Landscape.

Roman Dacia (/ˈdeɪʃə/ DAY-shə; also known as Dacia Traiana (Latin for 'Trajan’s Dacia'); or Dacia Felix, lit. 'Fertile Dacia') was a province of the Roman Empire from 106 to 271–275 AD. Its territory consisted of what are now the regions of Oltenia, Transylvania and Banat (today all in Romania, except the last region which is split among Romania, Hungary, and Serbia). During Roman rule, it was organized as an imperial province on the borders of the empire. It is estimated that the population of Roman Dacia ranged from 650,000 to 1,200,000. It was conquered by Trajan (98–117) after two campaigns that devastated the Dacian Kingdom of Decebalus. However, the Romans did not occupy its entirety; Crișana, Maramureș, and most of Moldavia remained under the Free Dacians.

After its integration into the empire, Roman Dacia saw frequent administrative reorganization. In 119 under Hadrian, it was divided into two departments: Dacia Superior ("Upper Dacia") and Dacia Inferior ("Lower Dacia"; later named Dacia Malvensis). Between 124 and around 158, Dacia Superior was divided into two provinces, Dacia Apulensis and Dacia Porolissensis. The three provinces would later be unified in 166 and be known as Tres Daciae ("Three Dacias") due to the ongoing Marcomannic Wars. New mines were opened and ore extraction intensified, while agriculture, stock breeding, and commerce flourished in the province. Roman Dacia was of great importance to the military stationed throughout the Balkans and became an urban province, with about ten cities known and all of them originating from old military camps. Eight of these held the highest rank of colonia. Ulpia Traiana Sarmizegetusa was the financial, religious, and legislative center and where the imperial procurator (finance officer) had his seat, while Apulum was Roman Dacia's military center.

In Romanian historiography, the Great Union (Romanian: Marea Unire) or Great Union of 1918 (Marea Unire din 1918) was the series of political unifications the Kingdom of Romania had with several of the Romanian historical regions, starting with Bessarabia on 27 March 1918, continuing with Bukovina on 28 November 1918 and finalizing with Transylvania (on its broad meaning) on 1 December 1918 with the declaration of the union of this region with Romania during an assembly at the city of Alba Iulia. Romanians also consider several other events as preludes to the Great Union, such as the unification of Moldavia and Wallachia (also known as the Little Union, Mica Unire) in 1859 or the independence of the country and the annexation of Northern Dobruja in 1878, and also the occupation of Transylvania and Moldavia by the Prince of Wallachia, Michael the Brave, in 1600.

Today, the Great Union has an important meaning in Romania, and it is commemorated in the Great Union Day, the national day of the country, every 1 December. The centenary of the Great Union on 1 December 2018 was widely celebrated in Romania, with military parades at cities like Alba Iulia and Bucharest to which many people (up to 550,000, 100,000 of them in Alba Iulia alone), including the President of Romania Klaus Iohannis, attended. The centenary was also celebrated in Moldova, where more than 100 localities and 3 districts declared unification with Romania.

The union of Transylvania with Romania was declared on 1 December [O.S. 18 November] 1918 by the assembly of the delegates of ethnic Romanians held in Alba Iulia. The Great Union Day (also called Unification Day), celebrated on 1 December, is a national holiday in Romania that celebrates this event. The holiday was established after the Romanian Revolution, and celebrates the unification not only of Transylvania, but also of Bessarabia and Bukovina and parts of Banat, Crișana and Maramureș with the Romanian Kingdom. Bessarabia and Bukovina had joined with the Kingdom of Romania earlier in 1918.

Michael the Brave (Romanian: Mihai Viteazul [miˈhaj viˈte̯azul] or Mihai Bravu [ˈbravu]; 1558 – 9 August 1601), born as Mihai Pătrașcu, was the Prince of Wallachia (as Michael II, 1593–1601), Prince of Moldavia (1600) and de facto ruler of Transylvania (1599–1600). He is considered one of Romania's greatest national heroes. Since the 19th century, Michael the Brave has been regarded by Romanian nationalists as a symbol of Romanian unity, as his reign marked the first time in history all principalities inhabited by Romanians were under the same ruler.

His rule over Wallachia began in the autumn of 1593. Two years later, war with the Ottomans began, a conflict in which the Prince fought the Battle of Călugăreni, resulting in a victory against an army nearly three times the size of the army of Michael the Brave, considered one of the most important battles of his reign. Although the Wallachians emerged victorious from the battle, Michael was forced to retreat with his troops and wait for aid from his allies, Prince Sigismund Báthory of Transylvania and Holy Roman Emperor Rudolf II. The war continued until a peace finally emerged in January 1597, but this lasted for only a year and a half. Peace was again reached in late 1599, when Michael was unable to continue the war due to lack of support from his allies. In 1599, Michael won the Battle of Șelimbăr against Andrew Báthory and soon entered Gyulafehérvár (today Alba Iulia, Romania), becoming the imperial governor (i.e. de facto ruler) of Transylvania, under Habsburg suzerainty. A few months later, Michael's troops invaded Moldavia and reached its capital, Iași. The Moldavian leader Ieremia Movilă fled to Poland and Michael was declared Prince of Moldavia. During this period, Michael the Brave changed his seal to represent his personal union of Wallachia, Moldavia, and Transylvania.

The Mureș (Romanian: [ˈmureʃ]) or Maros (Hungarian: [ˈmɒroʃ]; German: Mieresch, Serbian: Мориш / Moriš) is a 789-kilometre-long (490 mi) river in Eastern Europe. Its drainage basin covers an area of 30,332 km (11,711 sq mi). It originates in the Hășmașu Mare Range in the Eastern Carpathian Mountains, Romania, rising close to the headwaters of the river Olt, and joins the Tisza at Szeged in southeastern Hungary. In Romania, its length is 761 km (473 mi) and its basin size is 27,890 km (10,770 sq mi).

The Mureș River flows through the Romanian counties Harghita, Mureș, Alba, Hunedoara, Arad and Timiș, and the Hungarian county Csongrád. The largest cities on the Mureș/Maros are Târgu Mureș, Alba Iulia, Deva and Arad in Romania as well as Makó and Szeged in Hungary.

Apulum was a legionary fortress in the Roman province of Dacia from the 2nd to 4th centuries AD, located in today's Alba Iulia, Romania.

It is the largest castrum in Romania, occupying 37.5 hectares (93 acres) (750 x 500 m). It was the base of the legion Legio XIII Gemina transferred there by Trajan to the newly conquered province of Dacia at the end of the war in 106. In the era of Hadrian (117-138 AD) and of Antoninus Pius (138-161 AD) it was rebuilt in stone.

The Great National Assembly of Alba Iulia (Romanian: Marea Adunare Națională de la Alba Iulia) was an assembly held on 1 December 1918 in the city of Alba Iulia in which a total of 1,228 delegates from several areas inhabited by ethnic Romanians declared the union of Transylvania with Romania. It was summoned by the Romanian National Council. Regular ethnic Romanian civilians were also called to participate, and these came from all regions inhabited by Romanians; in total, the assembly was attended by some 100,000 people. The union of Transylvania with Romania was declared with the adoption of the Declaration of Alba Iulia (ro) during the assembly.

Although the assembly was announced for 1 December, debates on Transylvania's accession into Romania between prominent representatives of the Romanian National Central Council started already on 30 November. At the debate, chaired by Ștefan Cicio Pop, the present social democrats, including Ioan Flueraș, argued in favour of autonomy for Transylvania within Greater Romania, while the majority of nationalists and representatives of expatriates argued against autonomy and in favour of unconditional annexation.

Great Union Day (Romanian: Ziua Marii Uniri; also called Unification Day or National Day) is a Romanian national holiday celebrated on 1 December to mark the 1918 Great Union (the unification of Transylvania, Bassarabia, and Bukovina with the Romanian Kingdom). The holiday was declared after the Romanian revolution and commemorates the Great National Assembly of the delegates of ethnic Romanians held in Alba Iulia, who declared the Union of Transylvania with Romania.

Until the abolition of the Romanian monarchy in 1947, the national holiday was observed on 10 May, which had a double meaning as it was the date on which the future King Carol I first set foot on Romanian soil in 1866 and on which he later ratified Romania's Declaration of Independence from the Ottoman Empire in 1877. During the communist regime from 1947 to 1989, the national holiday was observed on 23 August (Liberation from Fascist Occupation Day) to mark the 1944 overthrow of Ion Antonescu's government by King Michael I, with parades held in Charles de Gaulle Square (then called Stalin Square and later Aviators' Square). In 1990, the holiday's date was moved to 1 December to match the date of the Great Union.