The main focus of this thesis is how the crusaders of the Fourth Crusade and its author, pope Inocent III. justified the conquest of Christian towns. The thesis is comprised of two parts. In the first and shorter part main focus will be on the just war theory by Thomas Aquinas. Just war theory is a theory whose purpose is to offer a justification of war. It presents a set of rules and guidelines on why and how to wage war in order for it to be just. The second and significantly larger part of this thesis follows the the crusader army giving the historical overview of the Fourth Crusade. In this part the just war theory is applied to the Fourth Crusade, specifically the actions of the crusader army during the war. During the negotiations with the doge Enrico Dandolo of the Venetian Republic for transport of the crusaders to Egypt , crusader emissaries overestimated their numbers. It was a mistake which the Venetians will skilfully exploit. Theobald, the leader of the crusade, soon died and was replaced by Boniface de Montferrat, a powerful Italian nobleman with strong ties to Byzantium. A year later, when the time came for the crusader army to pay its debt, it was unable to do so. Enrico Dandolo used this to pressure the crusaders into conquering the Christian city of Zadar for him despite pope's warnings that should they attack any Christian city they will all be excommunicated. After the city fell, the crusaders spent the winter there. It was there that they made the treaty with Alexius Angelus to help him restore his father Isaac II. to the Byzantine throne, from which he was previously overthrown by his brother. Crusaders did this but the relationship between them and Alexius soon deteriorated. This eventually led to an all out war between the crusaders and the Byzantine Empire in which the Empire lost. Crusader sacked the city of Constantinople bringing an end to the Byzantine Empire and forming their own Latin Empire in its ruins. What was supposed to be the greatest crusade against Saracens ended without having met a single one and destroying a Christian state in the process.