Of all the 15th century Ottoman battles in the Balkans (or Rumelia, as the Ottomans called their European possessions) prior to the conquest of Constantinople, the most important was probably the Battle of Varna in 1444, particularly if you pair its effects with those of the (second) Battle of Kosovo in 1448. Varna broke a major Hungarian-Polish alliance that had been formed to counter the Ottoman threat, so major in fact that it had been given the Crusader imprimatur (the “Crusade of Varna,” also known as “the Long Campaign”) by Pope Eugenius IV (d. 1447). The Ottoman victory here, combined with Kosovo, suppressed the Hungarian threat long enough to give the Ottomans time to focus on Constantinople, the big prize.
The Crusade of Varna, or the Long Campaign if you prefer, was a showcase for a military leader we’ve encountered already: Hungarian general John Hunyadi (d. 1456). Hungary and the Ottomans reached a peace treaty in the 1420s that left the rump Serbian state as a buffer between them, but in the 1430s the Ottomans, under Sultan Murad II (d. 1451), swallowed up Serbia as far north as Belgrade (which at the time was actually–briefly–Hungarian property), before they had to refocus their attention on fighting the Karamanids, their biggest Anatolian enemy.
The Hungarians underwent a succession crisis when Sigismund, who had been King of Hungary for 50 years, died in 1437, and his successor and son in-law, Albert (who was technically the first Habsburg ruler of Hungary, although the Habsburgs subsequently lost Hungary in 1457 and didn’t get it back until 1526), died in 1439 without an heir (his son, the future Ladislaus V–d. 1457–was born a few months after he died). Into this breach stepped Władysław III of Poland (d. 1444, FORESHADOWING), who, with the support of most of Hungary’s nobles and with John Hunyadi’s help, was crowned Vladislaus I of Hungary in 1440 (the infant Ladislaus V was crowned the same year and the throne was disputed until Varna, um, cleared things up). For his good work, Hunyadi was made commander of Hungarian forces in the south, and he and Vladislaus, along with Đurađ Branković, the Despot of Serbia (or whatever remained of it), resolved to take up Eugenius’s call for Crusade.
Murad initially had real problems responding to the Crusade. His attention was still partly diverted to the east, and he seems to have had a hard time raising enough reliable troops to put together a new army in Rumelia. The Hungarians in particular were well-armed and, under Hunyadi, very well-led, deploying tactics such as the laager (see here) to great success in disrupting Ottoman cavalry tactics. The Crusaders won a significant victory at Nish (in modern Serbia) in November 1443, but then overextended themselves in the face of an Ottoman scorched earth campaign and were defeated by Murad at the Battle of Zlatica (in modern Bulgaria) in December. In the midst of a general retreat back to Hungarian territory to hole up for the winter, they managed to win another victory over an Ottoman force at the Battle of Kunovica (again in modern Serbia) in January 1444. The campaign having ended on a high note, the Hungarian-Polish forces declared victory and waited for the arrival of spring and the new campaigning season.
Murad, though, appears to have had other ideas–he wanted…peace (you thought I was going to say revenge, admit it). He and Vladislaus traded envoys, and in August 1444 they agreed on the Peace of Szeged. Under the treaty, Murad was obliged to restore the Despotate of Serbia, along with considerable Serbian territory, to Branković, who was in turn obliged to make himself an Ottoman vassal. Hungary and the Ottomans agreed to 10 years of peace. As an aside, another part of the deal involved the Hungarian governor of Wallachia (in modern Romania), who had previously entered into an alliance with the Ottomans, possibly at some strenuous Ottoman urging. His obligations under that alliance were loosened, though he promised to pay annual tribute to the Ottomans as well as to send them conscripts. He also left two of his sons, who were already in Ottoman custody (being used as leverage to keep Wallachia from joining Hunyadi’s crusade), with the Ottomans as hostages. The governor’s name was Vlad II Dracul, and one of the sons he sent to the Ottoman court was the future Vlad III, AKA the Dracula.
Part of the reason Murad desired peace became apparent when he suddenly abdicated shortly after the treaty was concluded. I’ve written at length about this decision twice now, so I’ll spare you another turn except to note that his successor was his very young (~12 years old) son Mehmed II (d. 1481). What we should talk about here is the fact that Vladislaus and Hunyadi seem to have negotiated the Peace of Szeged without the slightest intention of sticking to its terms. Eugenius was very insistent that the Crusade continue, see, especially since it had been so successful in the previous fall/winter, and Vladislaus was able to use the campaign to quell support for Ladislaus and unite the Hungarian nobility behind his claim to the throne. Hunyadi, meanwhile, was convinced that the time to strike the Ottomans was now, and that 10 years of peace would only allow Murad to strengthen his position — plus, Vladislaus offered to make him King of Bulgaria if he repudiated the treaty and kept fighting, which appealed to Hunyadi very much. When they found out that Murad had abdicated in favor of a child, their resolve to keep fighting only grew. The only prominent member of the initial Crusade who didn’t want to keep fighting was Branković, who was quite happy with the terms of Szeged and therefore left the Crusader coalition. Eugenius sent a papal representative to Szeged to absolve Vladislaus of any obligation to uphold his treaty obligations, and that was that.
If you’ve been clicking on the links so far, you know what happened to the Ottomans when they found out that the Crusader army was approaching them again: Mehmed, at the, ah, strong urging of his army, demanded that his father unretire himself and command the Ottoman army in the coming campaign. Murad reluctantly agreed. The Crusader army, a combination of mostly Hungarian, Polish, Bohemian, and Wallachian troops, numbered between 20,000 and 30,000 men, against an Ottoman army that was probably twice that size (50,000-60,000), but less than half of it was made up of his reliable Janissary infantry and regular Sipahi cavalry. The Crusaders marched to Varna, on the western Black Sea coast (in modern Bulgaria), intending to meet a Venetian fleet and sail directly to Constantinople. Murad, coming from the west, managed to get to Varna before the fleet and trapped the Crusaders between his army and the Black Sea, with Lake Varna to the south and rugged terrain to the north locking the Crusaders in place.
Some of the Crusaders suggested a holding action, setting up their laager to keep the Ottomans at bay until the fleet could arrive. But Vladislaus, who was only 20 and wanted to be daring and bold, and Hunyadi, who was almost 40 and probably should have known better, pushed for an attack. They assembled their forces outside of Varna on the morning of November 10, with the Ottomans assuming a more defensive position behind ditches and field works (not unlike the laager, a tactic that the Janissaries were in the process of adopting for themselves). Hungarian firepower did considerable damage to the Ottoman cavalry, and the Crusaders nearly won the day, until Vladislaus made the mistake of taking some initiative. At some point, Hunyadi left their shared center position to lead reinforcements to their left flank and told Vladislaus to stay put until he returned. But Vladislaus (again, he was 20) decided instead to lead 500 of his best knights in a cavalry charge directly at Murad’s command post. As often happens in these situations, an act that could have decisively won the battle for one side (in this case, the Crusaders) wound up doing exactly the opposite, when Vladislaus was unhorsed and killed. Hunyadi tried to rally the troops to recover his body, but the demoralized army broke and ran instead.
The battle was not a one-sided rout, however; it’s said that Murad didn’t realize the Ottomans had won for days, so heavy were the Ottoman casualties. But Vladislaus’s death destroyed the Hungarian-Polish alliance that had fueled the Crusade; both Poland and Hungary fell into discord. Vladislaus was eventually succeeded in Poland by his brother, Casimir IV, who was already the Grand Duke of Lithuania, and in Hungary he was succeeded by Ladislaus, now Ladislaus V, but only after it was agreed that John Hunyadi would serve as his regent. Hunyadi took another bite at the Ottoman apple but was again decisively beaten at that Second Battle of Kosovo, which really took the final steam out of the Hungarians and quelled virtually all European resistance to the Ottomans for a time. The Hungarians would eventually resume their conflict with the Ottomans, but not until after Constantinople had fallen.