What passages of Scripture were used by the popes and crusaders to justify the Crusades?

St. Bernard of Clairvaux is perhaps the most well known promoter of the Crusades. He is credited for sparking the 2nd Crusade by writing very convincing letters to the Kings of Christendom. On Christmas Day, 1144, the Seljuk Turks captured Edessa, chief city of one of the Christian principalities set up by the First Crusade. Appeals for help went at once to Europe, for the position of all Christians in Syria was jeopardized. King Louis VII of France announced his intention of leading a new crusade, and the Pope commissioned Bernard to preach the Holy War.

Bernard sent a number of theologically charged letters to the various Kings and Queens of Christendom asking for support on behalf of the Church. His main selling point was the biblically sound cause of liberating the Jews from Muslim oppression, and also for their protection from rogue Crusaders.

In a letter written in 1146AD to Eastern France and Bavaria, Bernard promotes the 2nd Crusade by appealing to the role of Jews in God's plan of salvation:

Besides, brethren, I warn you, and not only I, but God's apostle, "Believe not every spirit." We have heard and rejoice that the zeal of God abounds in you, but it behooves no mind to be wanting in wisdom. The Jews must not be persecuted, slaughtered, nor even driven out. Inquire of the pages of Holy Writ. I know what is written in the Psalms as prophecy about the Jews. "God hath commanded me," says the Church, "Slay them not, lest my people forget."

They are living signs to use, representing the Lord's passion. For this reason they are dispersed into all regions, that now they may pay the just penalty of so great a crime, and that they may be witnesses of our redemption. Wherefore the Church, speaking in the same Psalm, says, "Scatter them by thy power; and bring them down, O Lord, our shield." So has it been. They have been dispersed, cast down. They undergo a hard captivity under Christian princes. Yet they shall be converted at even time, and remembrance of them shall be made in due season. Finally, when the multitude of the Gentiles shall have entered in, then, "all Israel shall be saved," saith the apostle. Meanwhile he who dies remains in death.

Elsewhere, he writes to the Archbishop of Mainz:

The fellow you mention in your letter [a monk named Raoul, who had urged violence against Jews as the Second Crusade was being organized] has received no authority from men or through men, nor has he been sent by God.

Does he consider himself greater than our father Abraham who laid down his sword at the bidding of him by whose command he took it up? Does he consider himself greater than the Prince of the Apostles who asked the Lord: "Shall we strike with our swords?" He is a fellow full of the wisdom of Egypt which is, as we know, foolishness in the sight of God. He is a fellow who answers Peter's question differently to the Lord who said: "Put back thy sword into its place; all those who take up the sword will perish by the sword." Is it not a far better triumph for the Church to convince and convert the Jews than to put them all to the sword? Has that prayer which the Church offers for the Jews, from the rising up of the sun to the going down thereof; that the veil may be taken from their hearts so that they may be led from the darkness of error into the light of truth, been instituted in vain? If she did not hope that they would believe and be converted, it would seem useless and vain for her to pray for them. But with the eye of mercy she considers how the Lord regards with favor him who renders good for evil and love for hatred. Otherwise where does that saying come in, "Not for their destruction I pray," and "When the fullness of the Gentiles shall have come in, then all Israel will be saved," and "The Lord is rebuilding Jerusalem, calling the banished sons of Israel home"? Who is this man that he should make out the Prophet to be a liar and render void the treasures of Christ's love and pity? This doctrine is not his own but his father's. But I believe it is good enough for him, since he is like his father who was, we know, "from the first a murderer, a liar, and the father of lies."

Of course we know from history that the 2nd Crusade was an overall failure. It is debatable whether or not the Pope's commission for St. Bernard to preach this failure was a good idea. Whatever the case may be, St. Bernard carried out his mission as a loyal servant to both the Church and to Sacred Scripture, ultimately becoming a canonized Doctor of the Church.

The Crusades began with Pope Urban II's speech at the Church Council at Clermont in 1095. The Crusades, as institutionalized Holy War with papal sanction against the perceived enemies of Christendom, ended in 1798 with the expulsion of the Hospital of St John from Malta by Napoleon.

St. Bernard's story is just one little chapter in a long book...