Introduction
There were a number of military orders founded in the late medieval and early modern periods. Often, these orders had some sort of crusading element to them. The primary focus of my answer will regard the Order of the Dragon, which was a federation of monarchs, barons, and other leaders who sought to halt the advance of the Ottoman Empire in the Balkans. However, there were several other orders founded in the 1400s.
For example, the Portuguese "Order of the Tower and Sword" of 1459, which honored those who participated in the battles of North Africa. Or the short-lived Latin "Order of Our Lady of Bethlehem" of 1459, which sought to confront the Ottoman Empire in Lemnos. Or the "Order of Saint George of Carinthia," which was tasked with defending Austria from the Ottomans. And, finally, the primary topic of this answer, the Order of the Dragon, introduced earlier.
Besides winning "Most Badass Name," the Order of the Dragon was actually an effective roadblock to Ottoman expansion in the Balkans for some time. And, as we will see, the Order is even connected to the historical Dracula, Vlad the Impaler.
Roadmap
First, I suggest that we hone in on the Order of the Dragon and look at exactly what its mission was. Second, we can look at its role in confronting the Ottomans. Finally we can look at its decline. Also, as a bit of added flavor, I'm going to detail how the establishment of this order influenced the rise of Vlad the Impaler and his triumphant and horrifying success against the Ottomans.
The Order of the Dragon
The Order of the Dragon was founded in 1408 by the King of Hungary, Sigismund of Luxembourg. Its 1408 charter states:
in company with the prelates, barons, and magnates of our kingdom, whom we invite to participate with us in this party, by reason of the sign and effigy of our pure inclination and intention to crush the pernicious deeds of the same perfidious Enemy, and of the followers of the ancient Dragon, and (as one would expect) of the pagan knights, schismatics, and other nations of the Orthodox faith, and those envious of the Cross of Christ, and of our kingdoms, and of his holy and saving religion of faith, under the banner of the triumphant Cross of Christ…^(1)
We should be careful to note that the Ottoman Empire wasn't the only "perfidious Enemy" of the Order of the Dragon. As outline further, heretics, heathens, and truly any enemy of the Hungarian state were also targets of the Order. For the sake of this answer though, I am going to focus on it's role in stalling the rapid expansion of the Ottoman Empire.
The emphasis on the Hungary kingdom and its protection stems from the fact that much of the early members would have been lords of the Hungarian realm. It wasn't until later that the Order of the Dragon expanded to include foreigners, such as the Wallachian prince, Vlad II, in 1431.
Actually, it is from this Order, that Vlad II gained his name, Vlad II Dracul. "Dracul" being "dragon" in Romanian. It was perhaps Vlad II's commitment to the Order that earned him his name. For he always wore the Order's amulet around his neck, marked money with its seal, and decorated a church and clock tower with its dragon.^(2) This is, of course, where his son, the legendary Vlad III Dracula derives his name, meaning "son of the dragon." So, there's a fun fact you can use around Halloween.
Besides, Vlad II Dracul, the Order was made up of a wide variety of foreign rulers over its lifetime, including, but not limited to: Gjergj Kastrioti (Skanderbeg) of Albania, Alfonso V of Aragon, Christopher of Bavaria, Vladislas II of Bohemia and Hungary, Fruzhin of Bulgaria, Hermann II of Cilli, Frederick II of Cilli, Hrvoje Vukčić Hrvatinić of Croatia and Bosnia, Henry V of England, Vytautas of Lithuania, Ferdinand I of Naples, Vladislav Jagello of Poland, Erik of Pomerania, Stefan Lazarević of Serbia, and Vuk Lazarević of Serbia.
This international alliance ensured Hungary would receive assistance that it needed in an offensive or defensive campaign against the Ottoman Empire. We have examples of members giving money, supplies, and soldiers to the Hungarian cause. Ultimately, through this assistance, one of the primarily goals of the Order was to liberate the Balkans and to extend Hungarian influence throughout it.