In 1095 Pope Urban II proclaimed the First Crusade at the Council of Clermont. He encouraged military support for the Byzantine Emperor Alexios I against the Seljuk Turks and an armed pilgrimage to Jerusalem. There was an enthusiastic response in western Europe across all social strata. https://en.wikipedia.org/wiki/Crusades
They were in direct response to Islam's liberation of the Holy lands
The Crusades were a series of religious wars initiated, supported and sometimes directed by the Latin Church in the medieval period, especially the campaigns between 1096 and 1271 in the Eastern Mediterranean aimed at recovering the Holy Land from Islamic rule. https://en.wikipedia.org/wiki/Crusades
The templar knights, who were christian warriors, got summoned
the Knights Templar or simply the Templars, were a Catholic military order founded in 1119 and recognised in 1139 by the papal bull Omne datum optimum.[4] The order was active until 1312 when it was perpetually suppressed by Pope Clement V by the bull Vox in excelso https://en.wikipedia.org/wiki/Knights_Templar
They were the elite fighting unit
Templar knights, in their distinctive white mantles with a red cross, were among the most skilled fighting units of the Crusades. https://en.wikipedia.org/wiki/Knights_Templar
As were troops from Britain, and elsewhere
The only significant Christian success of the Second Crusade came to a combined force of 13,000 Flemish, Frisian, Norman, English, Scottish, and German crusaders in 1147. Travelling from England, by ship, to the Holy Land, https://en.wikipedia.org/wiki/Second_Crusade
at the request of
Pope Eugene III https://en.wikipedia.org/wiki/Second_Crusade
and another involving English troops.
The Third Crusade (1189–1192) was an attempt by the leaders of the three most powerful states of Western Christianity (England, France and the Holy Roman Empire) to reconquer the Holy Land following the capture of Jerusalem by the Ayyubid sultan Saladin in 1187. https://en.wikipedia.org/wiki/Third_Crusade
But they could not prevent, Saladin from taking Jerusalem in 1187. He was that cocky, he offered to buy Jerusalem, in order to spare anymore bloodshed.
Saladin had captured almost every Crusader city. Saladin preferred to take Jerusalem without bloodshed and offered generous terms, but those inside refused to leave their holy city, vowing to destroy it in a fight to the death rather than see it handed over peacefully. Jerusalem capitulated to his forces on Friday, 2 October 1187 https://en.wikipedia.org/wiki/Saladin#C ... _Jerusalem
Next, Mehmed the conquerer came along a couple of hundred years later, after Islam had expanded in to Turkey, and he emerged victorious in yet another crusade battle
Mehmed the Conqueror (Turkish: Fatih Sultan Mehmet), was an Ottoman Sultan who ruled from August 1444 to September 1446, and then later from February 1451 to May 1481. In Mehmed II's first reign, he defeated the crusade led by John Hunyadi https://en.wikipedia.org/wiki/Mehmed_the_Conqueror
Now, this is not something that gets highlighted in western high schools, and is rather controversial.
There is arguments against this claim, such as the claim only being recognised by the patriarchate of constantinople.
The claim was only recognized by the Patriarchate of Constantinople. https://en.wikipedia.org/wiki/Mehmed_the_Conqueror
Or, contemporary scholar George of Trebizond, that supports his claim
The contemporary scholar George of Trebizond supported his claim https://en.wikipedia.org/wiki/Mehmed_th ... tantinople
And after doing what he did, he adopted the three children of the person he had just defeated, and left dead.
Emperor Constantine XI died without producing an heir, and had Constantinople not fallen to the Ottomans he likely would have been succeeded by the sons of his deceased elder brother. Those children were taken into the palace service of Mehmed after the fall of Constantinople. The oldest boy, renamed Has Murad, became a personal favorite of Mehmed and served as beylerbey of the Balkans. The younger son, renamed Mesih Pasha, became admiral of the Ottoman fleet and sanjak-bey of the Gallipoli. He eventually served twice as Grand Vizier under Mehmed's son, Bayezid II. https://en.wikipedia.org/wiki/Mehmed_th ... tantinople
And what he did was, defeat the last ever Christian Roman Emperor, and leave him dead, and adopt the 3 heirs to the throne.
Mehmed the conqueror aged 21, invaded the Roman empires capital of Constantinople, toppled what remained of the Roman empire, and declared himself "Roman Emperor". The first Muslim Roman emperor ever.
At the age of 21, he conquered Constantinople (modern-day Istanbul) and brought an end to the Byzantine Empire. After the conquest Mehmed claimed the title "Caesar" of the Roman Empire https://en.wikipedia.org/wiki/Mehmed_the_Conqueror