The Habsburg Monarchy (German: "Habsburgermonarchie" ) or Empire, is an unofficial appellation among historians for the countries and provinces that were ruled by the between 1521 and 1780 and then by the successor branch of Habsburg-Lorraine until 1918.
 The Monarchy was a composite state composed of territories within and outside the Holy Roman Empire, united only in the person of the monarch.
 The dynastic capital was Vienna, except from 1583 to 1611, when it was moved to Prague.
 From 1804 to 1867 the Habsburg Monarchy was formally unified as the Austrian Empire, and from 1867 to 1918 as the Austro-Hungarian Empire.
Jovan Šević or Ivan Šević (Serbian: Јован Шевић , Russian: Иван Егорович Шевич ; died  1764 ) was an 18th-century military officer of Serb origin.
 He reached the rank of lieutenant colonel in the Serb militia forces in the Pomorišje region, then in the Military Frontier of the Habsburg Monarchy.
 When it became obvious that privileges granted to Serb militiamen would be reduced or completely revoked after Pomorišje and Potisje lost their frontier status, Šević left Habsburg military service in 1750 and moved to Russia.
 At the end of 1752, he led the second wave of colonists who migrated from Pomorišje, Potisje and Slavonia to the Russian Empire (modern-day Ukraine) where they settled the newly established administrative region of Slavo-Serbia at the beginning of 1753.
 To enable him to recruit more of his fellow officers, Šević was promoted to the rank of General by the Russian Empress, Elizabeth.
 He commanded a Serb Hussar Regiment consisting of the colonists he brought to Russia.
 After Šević's death, Slavo-Serbia was disestablished, and many of his descendants became notable military officers in the Russian Imperial Army.
 Over time, all the Serb colonists became assimilated.
 Miloš Crnjanski described the migration led by Šević in his most notable work, the novel "Migrations" (Serbian: Сеобе ).
The Varaždin Generalate (German: "Warasdiner Generalat" , Croatian: "Varaždinski generalat" ), also known as the "Windische Grenze" ("Wendian/Wendish [Slavic] Border") in German, was a Habsburg Monarchy Military Frontier province centred in Warasdin (Varaždin), Kingdom of Croatia within Habsburg Monarchy, that existed between 1531 and the 18th century.
The Military Frontier was a province straddling the southern borderland of the Habsburg Monarchy and later the Austrian and Austro-Hungarian Empire.
 It acted as the "cordon sanitaire" against incursions from the Ottoman Empire.
The Kingdom of Croatia-Slavonia (Croatian: "Kraljevina Hrvatska i Slavonija" ; Hungarian: "Horvát-Szlavón Királyság" ; German: "Königreich Kroatien und Slawonien" ) was a nominally autonomous kingdom within the Austro-Hungarian Empire, created in 1868 by merging the kingdoms of Croatia and Slavonia following the Croatian–Hungarian Settlement.
 It was associated with the Hungarian Kingdom within the dual Austro-Hungarian state, being within the Lands of the Crown of St. Stephen or Transleithania.
 The kingdom was ruled by the Habsburg Emperor-King of Austria-Hungary ("Kaiser und König") under his title as "King of Croatia and Slavonia".
 The King's appointed steward was the Ban of Croatia and Slavonia.
 Although it was under the suzerainty of the Crown of Saint Stephen, the Kingdom of Croatia and Slavonia kept a significant level of self-rule.
The Slavonian Military Frontier (Croatian: "Slavonska vojna krajina" or "Slavonska vojna granica ") was a district of the Military Frontier, a territory in the Habsburg Monarchy, first during the period of the Austrian Empire and then during the Austro-Hungarian Monarchy.
 It was formed out of territories the Habsburgs conquered from the Ottoman Empire and included southern parts of Slavonia and Syrmia; today the area it covered is mostly in eastern Croatia, with its easternmost parts in northern Serbia (mostly in Vojvodina region).
Trenck's Pandurs (, German: "Panduren" ) were a light infantry unit of the Habsburg Monarchy, raised by Baron Franz von der Trenck under a charter issued by Maria Theresa of Austria in 1741.
 The unit was largely composed of volunteers from the Kingdom of Slavonia and Slavonian Military Frontier, and named after security guards otherwise employed to maintain public order.
 The Pandurs were presented to the empress in May 1741—with the unit's military band—earning them a claim of pioneering martial music in Europe.
 The Pandurs did not use uniforms and had an overall oriental/Ottoman appearance.
 The original organization of the unit was retained until 1745, when it transformed into a regiment.
 Trenck was relieved of command in 1746 and imprisoned in Spielberg Castle, where he died in 1749.
 The unit ultimately transformed into the 53rd Infantry Regiment, headquartered in Zagreb, until it was disbanded in 1919.
 The regiment's commemorative medals bear Trenck's image wearing Pandur attire.
The Kingdom of Slavonia (Croatian: "Kraljevina Slavonija" ; German: "Königreich Slawonien" ; Latin: "Regnum Sclavoniae" ; Hungarian: "Szlavón Királyság" ) was a province of the Habsburg Monarchy and the Austrian Empire that existed from 1699 to 1868.
 The province included northern parts of present-day regions of Slavonia (today in Croatia) and Syrmia (today in Serbia and Croatia).
 The southern parts of these regions were part of the Slavonian Military Frontier, which was a section of the Military Frontier.
Karlo Pavao Gvozdanović (German: "Karl Paul von Quosdanovich" , in some sources also "Quasdanovich" or "Guosdanovich"; Brezovac Žumberački near Samobor, Kingdom of Croatia within Habsburg Monarchy, 1763 – Pančevo, Banat Military Frontier, Habsburg Monarchy, 5 February 1817) was a Croatian nobleman and general in the Habsburg Monarchy imperial army service.
 He achieved the rank of major general and was awarded the Knight's Cross of the Military Order of Maria Theresa in 1801 and the Commander's Cross of the same order of merit in 1814.
Rascians (Latin: "Rasciani, Natio Rasciana" ) was an exonym in the early modern period that designated Serbs of the Habsburg Monarchy, and in a wider perspective other related South Slavic ethnic groups of the Monarchy, such as the Catholic Bunjevci and Šokci (designated "Catholic Rascians").
 The term was derived from the "Raška" (Rascia), a medieval Serbian region and exonym of the medieval Serbian state in Western sources.
 Because of the large concentration of Serbs in the southern Pannonian Plain, this region was called Rascia, today encompassing territories of Serbia (Vojvodina), Croatia (Slavonia, south Baranya and west Syrmia), Hungary (north Baranya and north Bácska) and Romania (east Banat and lower Marisus).

Question: What southern part of the Kingdom of Slavonia straddled the borderland of the Habsburg Monarchy?
Answer: The Military Frontier