Short answers, pulled from the story.
The Sack of Constantinople occurred in April 1204 and marked the culmination of the Fourth Crusade. The decisive assault on the city took place on the 12th of April 1204, and the looting that followed lasted three days.
Alexios IV Angelos persuaded the Crusaders to divert to Constantinople by promising 200,000 marks of silver, submission of the Eastern Orthodox Church to Rome, and provisions for the expedition. After Alexios IV was deposed and killed in early 1204, the Crusader and Venetian leadership formally agreed in March 1204 to conquer Constantinople outright in order to settle unpaid debts.
The total amount looted from Constantinople was said to be approximately 900,000 silver marks. Of that, 500,000 silver marks were reportedly stolen by Crusader knights; the Venetians received 150,000 silver marks and the Crusaders 50,000, with a further 100,000 divided evenly between the two groups.
The Latin Empire was established in Constantinople under Baldwin I of Flanders, while Byzantine aristocrats founded successor states including the Empire of Nicaea, the Empire of Trebizond, and the Despotate of Epirus. The Empire of Nicaea retook Constantinople in 1261, but the restored empire never recovered its former strength and ultimately fell to Ottoman Muslims in 1453.
Pope Innocent III did not sanction the sack and had explicitly forbidden the Crusaders from attacking the Byzantine Empire. When he learned of the events, he sent two angry letters to the crusade's leader, Boniface of Montferrat, condemning the looting of churches and the assault on Christian brothers.
Pope John Paul II twice expressed sorrow for the Fourth Crusade, first in a 2001 letter to Christodoulos, Archbishop of Athens, and again in 2004 when Patriarch Bartholomew I visited the Vatican. In April 2004, on the 800th anniversary of the sack, Ecumenical Patriarch Bartholomew I formally accepted the apology, stating "It is a fact that a crime was committed here in the city 800 years ago."