Final answer:
The two largest ethnic groups in the Balkans, historically and contemporary, are the Croats and Serbs, with Croats concentrated in Croatia and Serbs in Serbia.
Step-by-step explanation:
Based on historical information from the region of the Balkans, the two largest ethnic groups during the Austria-Hungary period and continuing afterward are likely the Croats (Croatians) and Serbs (Serbians).
The Croats are largely Roman Catholic and mainly reside in Croatia, while the Serbs are predominantly Eastern Orthodox Christians and mainly live in Serbia. These identifications hold true today as they did during the period of the Austro-Hungarian Empire and the subsequent time of Yugoslavia.
The Balkan Peninsula has been characterized by a variety of ethnic groups due to the complex history of invasions, empires, and nationalist movements. The breakup of Yugoslavia in the early 1990s, which was influenced by religious and national identities, resulted in distinct countries that align largely with ethnic lines. Understanding this historical context is essential for interpreting maps of ethnic groups in the area.