The clearest sign that crusading had become an instrument rather than a single campaign is the Fourth Crusade of 1202 to 1204, which never reached the Holy Land at all. Diverted first to the Christian city of Zara on the Adriatic at the demand of Venice, which had financed the fleet and wanted a rival port neutralized, the crusader army was then drawn into a succession dispute over the Byzantine throne. What began as an attempt to restore a deposed claimant ended in the storming and sack of Constantinople itself in April 1204, an Orthodox Christian capital, with churches looted, relics carried off to the West and a Latin emperor installed in place of the Byzantine one. The event poisoned relations between the Latin and Orthodox churches for centuries and is widely treated by historians as the clearest case of the crusading ideal outrunning its own stated purpose.
Later crusades kept widening the target list. The papacy authorized the Albigensian Crusade from 1209 against the Cathars of Languedoc in southern France, an internal war against a Christian heresy rather than a foreign campaign, and the Northern Crusades carried the same institutional machinery, preaching, vows and indulgences, against pagan peoples along the Baltic coast well into the fourteenth century. By the time the last mainland Crusader stronghold, Acre, fell to the Mamluks in 1291, the word crusade had already outgrown the Jerusalem campaign that coined it, and the practice of declaring one continued off and on for centuries afterward, invoked against the Ottoman advance into Europe and, in political rhetoric rather than the historical sense, into the present day.