And Makes Money
It pays to go your own way. At the time of the Crusades, Italy was not the one-nation of today. Instead, it was comprised of separate and rival city-states. That meant, among other things, that when a Pope in Rome declared a Crusade, the rest of Italy may or may not go along. That was the case here, where the calling for Crusades, on the supposedly religious principle of access for Christians to the Holy Land, brought in so many, but not Venice. Was that quest in the Holy Land, questionable as it may be as to motivation, in light of other motivations at the time (new power and profits to the newly separate Roman sect), worth sacrificing profits? Venice said no.
From the 9th Century to 1797, the Republic of Venice ran its own show and to great profit. See Republic of Venice, Arts, Other at http://www.metmuseum.org/toah/ht/?period=07®ion=eust
This interest of both sides in the power aspects of the Crusades suggests that rivalries among power areas was not entirely spiritual, including as to the Vatican.
There is a clear and concise summary of the Crusades, era est 1090-1290 AD, at http://history-world.org/crusades.htm. There were many crusades that the Pope ordered: against Muslims in the Middle East; and against any peoples who were not Latin-Church Roman Branch Christians, or where indeed "Christian" but opposed Papal supremacy. Once the Roman branch split with the Eastern Orthodox, who did not recognize the Pope as supreme, the game was on. The identity crisis began.
Crusades. The motivations for crusades were not just spiritual. There were nine crusades in the Middle East, see http://www.middle-ages.org.uk/crusades-timeline.htm, without reference to the Northern Crusades that were launched against Europeans of the north, and Slavic people already converted by Orthodox missionaries.
The Doge or ruler of Venice agreed, as a commercial matter, to transport Flemish and French soldiers to "Outremer", the general term for the Holy Land, "over the sea" in about 1199.
But when they got there, the soldiers did not have the money to pay. No pay, no travel. Venice asked that the soldiers substitute an attack on a rival of Venice, at Zara (now Zadar, Croatia) and then part of Hungary's influence. Zadar still is a sophisticated city. It is not difficult to imagine it as a rival to Venice for commerce. See it on Easter Morning with Cathedral renovations going on, at http://croatiaroadways.blogspot.com/2006/12/zadar-on-easter-morning.html
The Siege of Zara is well presented at Wikipedia, so there we go: 1202 AD -- Siege of Zara. This was Christian against Christian. Urbanity and sophistication against urbanity and sophistication.
And so they did. In 1202, the Venetians prevailed.
- But Innocent was not pleased at the diversion into Croatia, taking perhaps valuable time and momentum from his objective. He excommunicated the erstwhile Crusaders who had yet to fight the Muslims.
So, was Venice behind the Sack of Constantinople?
The Venetians, pleased with their success in steering the Crusaders to serve Venetian interests, persuaded them to attack Constantinople, the capital of the Byzantine Orthodox Christian world. Roman Christianity split with the rest of the Christians in The Great Schism of 1054, and was bolstering its identity as an independent and the authoritative branch of Christianity). The attack and sacking took three days, and Venice ended up with a nice monopoly on Byzantine trade.
Enter the Latin Empire of Constantinople, lasting untiil 1261 when the Byzantines recaptured their city.
The split between East and West Christianity was, perhaps, irrevocably broken. Did the old enmity extend into Rome's looking the other way in WWII when Orthodox were subject to genocide.
And so they did.
But why didn't the Crusaders then compel the Venetians to complete the bargain and transport them to the Holy Land? Or had they lost interest. Need more information.
Apparently, so far it appears that the participants were satisfied to skip it. Don't challenge the Muslims after all, but instead set up commercial ventures: various Crusader states along the way. See the history-world site.
Venice. The manipulator. Is that so? And does economic manipulation win? Usually.
The Uskoks, an ethnic group in Croatia, also found that Venice valued only its commercial interests when it came to fair dealing, see http://bogomilia.blogspot.com/2006/07/uskoks-at-senj-croatia.html