Ferdinand I, Holy Roman Emperor in the context of "Isabella Jagiellon"

Croatia, officially the Republic of Croatia, is a country in Central and Southeast Europe, on the coast of the Adriatic Sea. It borders Slovenia to the northwest, Hungary to the northeast, Serbia to the east, Bosnia and Herzegovina and Montenegro to the southeast, and shares a maritime border with Italy to the west. Its capital and largest city, Zagreb, forms one of the country's primary subdivisions, with twenty counties. Other major urban centers include Split, Rijeka and Osijek. The country spans 56,594 square kilometres (21,851 square miles), and has a population of nearly 3.9 million.

The Croats arrived in modern-day Croatia, then part of Roman Illyria, in the late 6th century. In the 7th century, they organized the territory into two duchies. Croatia was first internationally recognized as independent on 7 June 879 during the reign of Duke Branimir. Tomislav became the first king by 925, elevating Croatia to the status of a kingdom. During the succession crisis after the Trpimirović dynasty ended, Croatia entered a personal union with Hungary in 1102. In 1527, faced with Ottoman conquest, the Croatian Parliament elected Ferdinand I of Austria to the Croatian throne. In October 1918, the State of Slovenes, Croats, and Serbs, independent from the Habsburg Empire, was proclaimed in Zagreb, and in December 1918, it merged into the Kingdom of Yugoslavia. Following the Axis invasion of Yugoslavia in April 1941, most of Croatia was incorporated into a Nazi-installed puppet state, the Independent State of Croatia. A resistance movement led to the creation of the Socialist Republic of Croatia, which after the war became a founding member and constituent of the Socialist Federal Republic of Yugoslavia. On 25 June 1991, Croatia declared independence, and the War of Independence was successfully fought over the next four years.

The Habsburg monarchy, also known as Habsburg Empire, or Habsburg Realm (/ˈhæpsbɜːrɡ/), and Danube Monarchy, was the collection of empires, kingdoms, duchies, counties and other polities (composite monarchy) that were ruled by the House of Habsburg. From 1804 it was referred to as the Austrian Empire, and from 1867 as Austria-Hungary.

The history of the Habsburg monarchy can be traced back to the election of Rudolf I as King of Germany in 1273 and his acquisition of the Duchy of Austria for the Habsburgs in 1282. In 1482, Maximilian I acquired the Netherlands through marriage. Both realms passed to his grandson and successor, Charles V, who also inherited the Spanish throne and its colonial possessions, and thus came to rule the Habsburg empire at its greatest territorial extent. The abdication of Charles V in 1556 led to a division within the dynasty between his son Philip II of Spain and his brother Ferdinand I, who had served as his lieutenant and the elected king of Hungary, Croatia and Bohemia. The Spanish branch (which held all of Iberia, the Netherlands, and lands in Italy) became extinct in the male line in 1700, but continued through the female line through the House of Bourbon. The Austrian branch (which ruled the Holy Roman Empire, Hungary, Bohemia and various other lands) was itself split into different branches in 1564 but reunited 101 years later. It became extinct in the male line in 1740, but continued through the female line as the House of Habsburg-Lorraine.

The Kingdom of Hungary between 1526 and 1867 existed as a state outside the Holy Roman Empire, but part of the lands of the Habsburg monarchy that became the Austrian Empire in 1804. After the Battle of Mohács in 1526, the country was ruled by two crowned kings (John I and Ferdinand I). Initially, the exact territory under Habsburg rule was disputed because both rulers claimed the whole kingdom. This unsettled period lasted until 1570 when John Sigismund Zápolya (John II) abdicated as King of Hungary in Emperor Maximilian II's favor.

In the early stages, the lands that were ruled by the Habsburg Hungarian kings were regarded as both the "Kingdom of Hungary" and "Royal Hungary". Royal Hungary was the symbol of the continuity of formal law after the Ottoman occupation, because it could preserve its legal traditions, but in general, it was de facto a Habsburg province. The Hungarian nobility forced Vienna to admit that Hungary was a special unit of the Habsburg lands and had to be ruled in conformity with its own special laws. However, Hungarian historiography positioned Transylvania in a direct continuity with the medieval Kingdom of Hungary in pursuance of the advancement of Hungarian interests.

The siege of Vienna, in 1529, was the first attempt by the Ottoman Empire to capture the city of Vienna in the Archduchy of Austria, part of the Holy Roman Empire. Suleiman the Magnificent, sultan of the Ottomans, attacked the city with over 100,000 men, while the defenders, led by Niklas Graf Salm, numbered no more than 21,000. Nevertheless, Vienna was able to survive the siege, which ultimately lasted just over two weeks, from 27 September to 15 October 1529.

The siege came in the aftermath of the 1526 Battle of Mohács, which had resulted in the death of Louis II, King of Hungary, and the descent of the kingdom into civil war. Following Louis' death, rival factions within Hungary selected two successors: Archduke Ferdinand I of Austria, supported by the House of Habsburg, and John Zápolya. Zápolya would eventually seek aid from, and become a vassal of the Ottoman Empire, after Ferdinand began to take control of western Hungary, including the city of Buda.

The Lands of the Bohemian Crown were the states in Central Europe during the medieval and early modern periods with feudal obligations to the Bohemian kings. The crown lands primarily consisted of the Kingdom of Bohemia, an electorate of the Holy Roman Empire according to the Golden Bull of 1356, the Margraviate of Moravia, the duchies of Silesia, and the two Lusatias, known as the Margraviate of Upper Lusatia and the Margraviate of Lower Lusatia, as well as other territories throughout its history. This agglomeration of states nominally under the rule of the Bohemian kings was referred to simply as Bohemia.

The joint rule of Corona regni Bohemiae was legally established by decree of King Charles IV issued on 7 April 1348, on the foundation of the original Czech lands ruled by the Přemyslid dynasty until 1306. By linking the territories, the interconnection of crown lands thus no more belonged to a king or a dynasty but to the Bohemian monarchy itself, symbolized by the Crown of Saint Wenceslas. During the reign of King Ferdinand I from 1526, the lands of the Bohemian Crown became a constituent part of the Habsburg monarchy. A large part of Silesia was lost in the mid-18th century, but the rest of the Lands passed to the Austrian Empire and the Cisleithanian half of Austria-Hungary. By the Czechoslovak declaration of independence in 1918, the remaining Czech lands became part of the First Czechoslovak Republic.

The Erblande ("Hereditary Lands") of the House of Habsburg formed the Alpine heartland of the Habsburg monarchy. They were the hereditary possessions of the Habsburgs within the Holy Roman Empire from before 1526. The Erblande were not all unified under the head of the dynasty prior to the 17th century. They were divided into several groupings: the Archduchy of Austria, Inner Austria, the County of Tyrol, and Further Austria.

The Erblande did not include either the Lands of the Bohemian Crown or the Lands of the Hungarian Crown, since both monarchies were elective when the Habsburg Ferdinand I was elected to their thrones in 1526. Ferdinand divided the Erblande between his three heirs in 1564 and they were not reunited until 1665. The Erblande were gathered into the Austrian Circle in 1512. This ensured a direct connection between the junior lines of the Austrian Habsburgs and the Empire after 1564, since throughout this period the Austrian Habsburgs exercised only one vote in the Council of Princes.

The Empire of Charles V, also known as the Habsburg Empire, included the Habsburg hereditary lands in central Europe, the kingdoms of Spain, the colonial Spanish Empire, the kingdom of Naples, the Habsburg Netherlands and other territories and principalities across Europe. It is sometimes considered to include, in addition, the kingdoms of Bohemia and Hungary which were held by Charles's brother Ferdinand during his reign. Charles was also Holy Roman Emperor and, as such, was suzerain of the states of the Holy Roman Empire.

The empire was the first to be labelled as "the empire on which the sun never sets", a term used to describe several global empires throughout history. The lands of the empire had in common only the monarch, Charles V, while their boundaries, institutions, and laws remained distinct. Charles's nomenclature as Holy Roman Emperor was Charles V (also Karl V and Carolus V), though earlier in his life he was known by the names of Charles of Ghent (after his birthplace in Flanders), Charles II as Duke of Burgundy, and Charles I as King of Spain (Carlos I) and Archduke of Austria (Karl I). The imperial name prevailed due to the politico-religious primacy held by the Holy Roman Empire among European monarchies since the Middle Ages, which Charles V intended to preserve as part of his (ultimately failed) project to unite Christendom under his leadership.