When and how did the Crusades begin, and what was the role and relationship of Nur al-Din Zanki and Salah al-Din al-Ayyubi?

The Crusader campaigns against Muslims numbered seven, lasting nearly 200 years, beginning with the First Crusade (488-492 AH / 1095-1099 CE), which headed to Jerusalem and committed horrific massacres upon its entry, killing over seventy thousand. Among the Muslim leaders who resisted the Crusaders were Nur al-Din Zengi, who dedicated himself to fighting them and prepared a minbar for Al-Aqsa Mosque, and Salah al-Din al-Ayyubi, who was a commander in Nur al-Din's army, then became vizier in Egypt. At his hands, Jerusalem was liberated after the Battle of Hattin in 583 AH.