Yes but they aren't doing it now are they?
Are you saying that the Crusades were unjust?
To answer that question, first we must understand that the Church has never taught that all violence is evil or sinful. Divine Revelation affords the use of violence in certain cases and for just reasons.
The Old Testament is replete with examples of legitimate warfare sanctioned by God undertaken by the Jewish people. These examples clearly illustrate that God commanded and allowed the use of violence for a holy purpose.
Our Lord Himself overturned the tables of the money-changers in the Temple in Jerusalem.
The Crusades were born from the violent aggression of Islam, which had conquered ancient Christian territory in the Holy Land and North Africa and established a large foothold in Europe within a century of Muhammad’s death in the early seventh century.
The Crusades were also a response to the severe persecution of indigenous Christians living in the occupied territories, whose lives were severely restricted and who suffered constant pressure to convert to Islam. As an example, in the early eleventh century, Christians living in the Fatimid caliphate were subject to persecution during the reign of al-Hakim, who ordered them to wear identifying black turbans and a large cross in public. He also ordered the destruction of the Church of the Holy Sepulchre in Jerusalem, originally built by the Roman Emperor Constantine and St. Helena in the fourth century.
Christian pilgrims were also subjected to harassment and violence, which demanded a defensive response from Christendom. The Seljuks, who were known for their brutality, threatened pilgrims to the Holy Sites in Palestine. As an example, a group of 12,000 German pilgrims led by Bishop Günther of Bamberg in 1065 was massacred by the Seljuks on Good Friday, only two days’ march from Jerusalem.
Heroic men and women of faith, rooted in love of Christ and neighbour, undertook the Crusades as acts of self-defense and recovery of stolen property.
This is the proper understanding of these important events in Church history