In the century after the death of Osman I in 1326, Ottoman rule began to extend over the eastern Mediterranean and the Balkans, slowly at first and later in earnest.

Gallipoli was captured in 1354, severing the Byzantine Empire from its continental territories;[19] the important city of Thessaloniki[20] (with a greater population than London at the time) was captured from the Venetians in 1387, and the Turkish victory at the Battle of Kosovo in 1389 effectively marked the end of Serbian power in the region, paving the way for Ottoman expansion into the rest of Europe.

[citation needed] In 1344, Louis I of Hungary, who would rule from 1342–1382 and earn the epithet "the Great", invaded Wallachia and Moldavia and established a system of vassalage.

His latter campaigns in the Balkans were aimed not so much at conquest and subjugation as at drawing the Serbs, Bosnians, Wallachians and Bulgarians into the fold of the Roman Catholic faith and at forming a united front against the Turks.

[23] In 1366 Byzantine Emperor John V visited Hungary to beg for help against the Ottoman Turks, who were in increasing conflict with the Balkan vassal states.

[24] The Battle of Nicopolis (25 September 1396) is thought to be the first military encounter between Hungary and the Ottoman Empire, where a broad coalition force of Christian monarchs and European military commanders including the Knights Hospitaller was comprehensively annihilated by a numerically inferior Turkish army under the capable command of their 4th Sultan, Bayezid the Thunderbolt .

After a decade of internecine battles, Mehmed I emerged victorious and reestablished the Ottoman Empire and Byzantine Emperor accepted its vassalship and agreed to pay tribute.

[citation needed] With Byzantium no longer a threat, Murad II began his war against his Christian opponents, attacking Macedonia and capturing Thessalonika from the Venetians in 1430.

[citation needed] In the 1440s and 1450s, the Hungarian military leader John Hunyadi became the key architect of campaigns against the Ottoman Empire.

[28] In March 1442, Hunyadi defeated Mezid Bey and the raiding Ottoman army at the Battle of Szeben in the south part of the Kingdom of Hungary in Transylvania.

This was the first time that a European army defeated such a large Ottoman force, composed not only of raiders, but of the provincial cavalry led by their own sanjak beys (governors) and accompanied by the formidable janissaries.

[citation needed] With the help of knights from western Europe, Hunyadi succeeded in capturing Nis on November 3, 1443, defeating another Turkish army as they crossed the Balkan Mountains and then taking another victory on Christmas Day.

Meanwhile, the Ottoman issue had again become acute, and, after the fall of Constantinople in 1453, Sultan Mehmed II was rallying his resources to subjugate Hungary.

At his own expense, he restocked the supplies and arms of the fortress, leaving a strong garrison there under the command of his brother-in-law Mihály Szilágyi and his own eldest son Hunyadi László.

His main ally was the Franciscan friar, Giovanni da Capistrano, whose fiery oratory drew a large crusade made up mostly of peasants.

Although relatively ill-armed (most were armed with farm equipment, such as scythes and pitchforks) they flocked to Hunyadi and his small corps of seasoned mercenaries and cavalry.

"[citation needed] During the battle, Pope Callixtus III had ordered the bells of every European church to be rung every day at noon, as a call for believers to pray for the defenders of Belgrade.

In 1479 an Ottoman army, on its return home from ravaging Transylvania, was annihilated at Szászváros (modern Orăştie, 13 October 1479) in the Battle of Breadfield.

Mehmed II's post-Constantinople troubles escalated further when the principality of Wallachia under Count Vlad III Dracul rebelled against the Ottoman Empire and declared the King of Hungary as his suzerain.

In 1461, five years after his return, Vlad initiated war with the Turks when he impaled the Turkish ambassadors demanding tribute from him and took the fortress of Giurgiu.

Vlad then began a bloody assault across the Danube to the Black Sea, destroying as many of the ports as he could to prevent Ottoman naval attacks.

After a failed attack on the Ottoman vanguard Stephen seemed on the brink of defeat when King Matthias Corvinus of Hungary offered assistance.

Suleiman the Magnificent resumed the war against Hungary by attacking the city of Belgrade, the same settlement that had defied Mehmed II over half a century earlier.

When Suleiman launched an invasion in 1526 the Grand Vizier constructed a great bridge ahead of the Sultan allowing his army to march into Hungary.

With the cavalry annihilated, the infantry suffered immense casualties as the weight of numbers of the Ottomans and their skill in battle took their toll.

John Zápolya, who had been instructed by Louis II to raid the enemy's supply lines, arrived at the battle too late and fled the scene.

Suleiman, however, was not ready to annex the Kingdom completely into the Ottoman realm and in that power gap, Zapolya was chosen by Hungarian electorate as their ruler.

The eastern part of the kingdom (Partium and Transylvania) became at first an independent principality, but gradually was brought under Turkish rule as a vassal state of the Ottoman Empire.