The Crusades, when rightly estimated, appear to have been just and lawful enterprises,
since they were organised in order to protect the Christians of the East from the cruel oppression of the Mahometans, and to defend Europe against the fury of the barbarians, who were threatening a universal invasion.
These gigantic expeditions should be judged of in their entirety,. without considering faults and abuses, which accompany all wars. The ill-success of most of the expeditions is to be attributed to the perfidy of the Greeks, and to the disorders which had too commonly crept in amongst the Crusaders.
If these wars failed in their original design of delivering the East from the grasp of infidelity, they were, on the other hand, productive of most advantageous results in the West.
They preserved Christianity and civilization from Muslim invasion; they delivered the people of Europe from evils which they had caused themselves; they put an end to the wars between Christian kings, and they extinguished that civil discord which for two centuries had kept the feudal lords in arms against each other.
Moreover, they improved the condition of the people, who became free by taking part in these glorious expeditions; and, in augmenting the influence of the Popes, who were the natural protectors of nations, they provided social order with an efficacious safeguard.
It is also incontestable that they reawakened a taste for commerce, science, literature, and the arts, and that they prepared the way for the outbursts of genius which took place during the reigns of Leo X. and Louis XIV. of France.
We may add, that the Crusades were supported by the greatest and most saintly men of their time; that they were solemnly authorized by the Church, protected by the divine assistance,
which could not but aid her in such a grave matter, and that they were sanctioned more than once by miracles accompanying their publication.
Link (here) to the Belgian Jesuit. Fr. F. X. Schouppe, S.J. in his book entitled, Abridged Course of Religious Instruction.