Key Dates in Christendom
10 October 732 - the Battle of Poitiers (also called the Battle of Tours) - a decisive turning point in the struggle of Christendom against Islamic aggression. The Frankish and Burgundian forces under Charles Martel (Charles the Hammer/ Carolus Martellus (688-741), the father of Pepin the Short and the grandfather of Charlemagne) defeated the Islamic forces of the Umayyad Caliphate led by Abdul Rahman Al Ghafiqi, halting the Muslim advance into Christian Europe.
16 July 1054 - the nominal dating of the beginning of the Great Schism, when the legates of Pope Saint Leo IX (21 June 1002 - 19 April 1054, reigned 12 February 1049 - 19 April 1054) -- Humbert Cardinal of Silva Candida (1015-1061), Frederick of Lorraine, and Archbishop Peter of Amalfi -- entered the Cathedral of Hagia Sophia during the Divine Liturgy and placing a bull of excommunication on the altar, although the excommunication was directed only against the Patriarch of Constantinople Michael I Cerularius (1000-1059, reigned 1043-1059) and a few associates and not against the entire Constantinopolitan Patriarchate or the Byzantine Church and although it was issued after the death of Pope Leo IX on 19 April 1054 which event deprived the legates of authority to excommunicate anyone in the name of the Roman Patriarchate.
April 1182 - the Massacre of the Latins - the murder or expulsion of 60,000 Latin Christians (mostly Venitians, Pisans and Genoese) by the Greek Christian citizenry in Constantinople under just-installed Emperor Andronikos I Komnenos.
[3 days beginning circa 12] April 1204 - the attack on and capture and sack of Constantinople, the ugly event of the Fourth Crusade (originally intended to restore access for Christians to the Holy Land) and one of the most destructive events between western and eastern Christians.
12 September 1683 - the Battle of Vienna / Schlacht am Kahlenberg / Bitwa pod Wiedniem / Віденська відсіч - a decisive turning point in the struggle of European Christendom against Islamic aggression. The Holy League of approximately 84,000 Polish-Lithuanian, Austrian and German forces commanded by King of Poland Jan III Sobieski defeated the Muslim Ottoman Turk forces of no less than 150,000 and possibly as many as 300,000, which had besieged Vienna since 14 July 1683. The Muslim Turkish commander, Grand Vizier Merzifonlu Kara Mustafa Pasha, was later killed by his Turkish countrymen on 25 December 1683. Over the ensuing years the Austrians cleared the occupying Muslim Turks from Hungary, Transylvania and portions of the Balkans.