The Siege of Zadar (November 11-November 23, 1202) was the first major action of the Fourth Crusade. It was the first attack against a Catholic city by Catholic crusaders.
Shortly after his election as pope in 1198, Pope Innocent III (1161-1216) published several papal encyclicals calling for the invasion and recapture of the Holy Land from the Muslims. His plan to accomplish this differed from the earlier ultimately unsuccessful Second and Third Crusades in several ways. Instead of the secular nobles who led the earlier crusades, this one would be, in theory, completely under Papal control. Innocent´s plan also called for the invading armies to travel to Egypt by sea and seize the Nile Delta, which would then be used as a base from which to invade Palestine. His call was at first poorly received among the ruling families of Europe, but by 1200 an army of approximately 35,000 soldiers had been assembled under Boniface of Montferrat.
Innocent III negotiated an agreement with the Republic of Venice, Europe´s dominant sea power at the time, involving the construction of a fleet of warships and transports, to be paid for at the time of their use. The deal stipulated that about 35,000 crusaders would need transport and the Venetians would be paid 85,000 silver marks. After the Venetians had suspended their commercial operations for a year to build and crew the ships, only about 12,000 showed up at Venice to man and pay for them. The crusaders thus found themselves only able to pay 51,000 marks to the Venetians. In response, the Venetians indicated that they would accept the invasion of Zadar, a Catholic city on the coast of the Adriatic, in lieu of payment for the time being; the crusaders were then to pay the rest owed to the Venetians out of their initial gains in the crusade. Zadar had rebelled against the Venetian Republic in 1183, and placed itself under the dual protection of the Papacy and King Emeric of Hungary (who had also recently happened to agree to join the crusade).
Though a large group of Crusaders found the scheme repulsive and refused to participate, the majority agreed (despite the written protests of Innocent III), citing it as the only means necessary to attain the larger goal of taking Jerusalem. In the winter of 1203, Innocent excommunicated the entire crusading army, along with the Venetians, for taking part in the attack. "Behold," the pope wrote, "your gold has turned into base metal and your silver has almost completely rusted since, departing from the purity of your plan and turning aside from the path onto the impassable road, you have, so to speak, withdrawn your hand from the plough [...] for when [...] you should have hastened to the land flowing with milk and honey, you turned away, going astray in the direction of the desert." Pope Innocent was to later grant an absolution to the entire army.
The attack on Zadar took the form of an amphibious landing followed by a brief siege. The incident was to foreshadow the Siege of Constantinople later in the campaign. The crusaders used the 50 amphibious transports, 100 horse carriers and 60 warships designed and built for them by the Venetians. Their transports were approximately 30 m long, 9 m wide and 12 m high, with a crew of 100. Each one could carry up to 600 footmen. The horse carriers featured specially designed slings to carry their cargo of horses, and featured a fold-out ramp below the waterline that could be opened to allow mounted knights to charge directly onto shore. The Venetian warships were powered by 100 oarsmen each and featured a metal-tipped ram just above the waterline as their primary weapon. Also, during the siege, 150 siege engines were used to bombard the city´s walls.
Chains and booms were laid across the mouth of Zadar´s harbor as a defense, but the Crusaders burst through them in their Venetian ships and landed their troops and equipment without harassment. Zadar fell on November 23, 1202.