In 1352, Ottomans set foot on the European continent in Gallipoli. Subsequently after a few years, Stefan Dushan died in 1355, and the Serbian state came to be divided into smaller parts. They were economically and, in some places, territorially dominated by the Western states of Hungary, Venice and Genoa.
Not only were the Balkans divided into small states, but every Balkan state was divided internally, into two factions: the pro-Latin faction which were mostly popular among the ruling classes, and the staunch anti-Latin Orthodox Christian faction supported by the peasantry.
By taking advantage of the state of affairs, Ottomans succeeded in the Balkans. However, the first problem posed before the Ottomans was to cross the sea to Europe without a navy. But this became possible for them owing to the alliance with Genoa, which was at war with Venice at that point of time.
In return the Ottomans helped Genoa in Gallipoli, and granted them flattering terms in trade. These were the first capitulations in Ottoman history. Once they were in Gallipoli, the Ottomans managed to stay in Europe because of their Byzantine ally and relative namely Kantakuzenos.
Having secured their foothold, the Ottomans began expanding in the Balkans, by taking appropriate sides in various Balkan conflicts, such as helping the Bulgarian king against an attack by Byzantium, Hungary and Wallachia. Their expansion followed the Roman road via Egnatia, which was the main trade route in the Balkans. Adrianople was captured in 1361, and was made the new capital.
This move signified the continuance of the Balkan expansion. As a result, Balkan cities fell one after the other. In 1371, a large Serbian army moved east, but was defeated by the Ottomans in Chirmen. After this battle, the Balkan states and Byzantium became vassals of the Ottoman sultan.
Ottoman's increasing dominance in the Balkans brought them into direct conflict with Hungary which was the strongest state bordering the region. Further attempts of driving the Ottomans back to Anatolia were made. But they failed. Ottomans somehow managed to protect their dominance in the Balkans by two spectacular victories, the first against a large Balkan alliance in Kosovo and the second against another large, this time Western crusader army in Nicopolis both fought in 1396.
The success of the Ottomans in defeating their enemies and subsequent 400 year rule over the infamous Balkans resulted in the growth of population of people belonging to different cultures and religions. In lands where they annexed, if the local nobles cooperated with them, the Ottomans allowed them to retain their noble status as well as their Christian faith. Hence in this period there were many Christians even from higher classes who willingly worked for the Ottomans.
It was not until later that the process of converting into Islam of the ruling classes commenced. Thus an extremely planned and well organised move was undertaken by the Ottomans which helped them not only in acquiring new territories but also to bring them under an uniformed Ottoman rule.