On the crusades themselves, I think your friends have been following modern revisionist histories too closely. The Crusades were a defensive war, called to protect fellow Christians from Muslim assault.
From 600 AD on, Islam has been an expanding religion, which has aimed to conquer the lands of Christianity. This continuous war or “jihad” began just three years after Muhammad’s death and continued continuously for the next thousand years. In this time the Muslim overran the Christian Lands of
Syria, Jordan, Lebanon and Palestine 630 AD
Egypt 650 AD
North Africa, Libya, Tunisia, Algeria, Morocco 700 AD
Spain 780 AD
Southern France 790 AD
Sicily 850 AD
Southern Italy 860 AD
Turkish borders 900 AD
Armenia and Georgia 1050 AD
Central Turkey 1070 AD
Greece 1300 AD
Bulgaria and the Balkans 1400 AD
Constantinople 1450 AD
Of all the New Testament Churches, only a single one, Rome, escaped Muslim domination.
Piratical raids and campaigns of raiding on land, slave-taking and slaughter took place virtually every summer for a thousand years to destabilise neighbouring Christian lands. No permanent peace with “infidels” was allowed by Islam.
The one time the Christians decided that enough was enough and united to fight back was the Crusades. And it is arguable that without them, Europe would be Muslim ruled today. The Crusades had two causes:
An appeal from the Byzanine Empire because the Muslims had broken through their defenses and laying waste to Christian Anatolia.
The systematic destruction of Christian sites in the Holy Land and the prevention of pilgrimage to Jerusalem.
This is from Pope Urban II’s call for a crusade at the Council of Cllermont in 1095
Freshly quickened by the divine correction, you must apply the strength of your righteousness to another matter which concerns you as well as God. For your brethren who live in the east are in urgent need of your help, and you must hasten to give them the aid which has often been promised them. For, as the most of you have heard, the Turks and Arabs have attacked them and have conquered the territory of Romania [the Greek empire] as far west as the shore of the Mediterranean and the Hellespont, which is called the Arm of St. George. They have occupied more and more of the lands of those Christians, and have overcome them in seven battles. They have killed and captured many, and have destroyed the churches and devastated the empire. If you permit them to continue thus for awhile with impurity, the faithful of God will be much more widely attacked by them. On this account I, or rather the Lord, beseech you as Christ’s heralds to publish this everywhere and to persuade all people of whatever rank, foot-soldiers and knights, poor and rich, to carry aid promptly to those Christians and to destroy that vile race from the lands of our friends. I say this to those who are present, it meant also for those who are absent. Moreover, Christ commands it.
Muslim attack on Armenia, 1059, 30 years **before ** the Crusades.
On Sunday 6th August the siege of Sebastea began, as did the slaughter; thousands of corpses littered the ground. What a dreadful scene. The bodies of highly renowned men were piled in a heap as if a forest of trees had been felled. and the ground was soaked with blood…
They ruthlessly massacred an immense number of people, carried off booty and took untold numbers of captives, men and women, young boys and girls, whom they sold into slavery… Fateful day! In a matter of minutes sebastea and the surrounding plain were bathed in blood. The clear waters of the River Kizil Irmak which cuts through the city walls, suddenly flowed red.
In 1064 the Turks returned. He made his way towards Armenia and entered the country; the inhabitants were put to the sword and driven into slavery. The infidels were so numerous that they covered the plains and closed off al the escape routes. Then he invaded Georgia, bringing death and slavery wherever he went. … The Turks exterminated all the inhabitants, men, women, priests, monks and nobles; the young boys and girls were taken away captive into Persia…" Chronicle of Matthew of Edessa.