Recently I came across the videos of Real Crusades History. While I am aware that this channel has been discussed on this subreddit before, I would like a bit of clarification on their stances. To sum up, they believe that the Crusaders were wars of defense on Europe's part. Stopping Muslim influx into Europe and ending oppression of Christians in Muslim territories. How much of this is true?
Crusades Saved Europe: https://www.youtube.com/watch?v=cMjUFBYEzqQ
Defensive War: https://www.youtube.com/watch?v=8Dl6U4SRhcY
Crusades in 5 Minutes: https://www.youtube.com/watch?v=CcGzQ3ga5R8
These videos play into one of two contradictory interpretations of the crusades. The first is that the crusades were unprovoked attacks by Western barbarians against a peaceful and civilised Greek and Islamic world. Steven Runciman's History of the Crusades is one of the most famous works taking this stance. The other extreme is that the crusades were long overdue responses to repeated Islamic aggression. Neither interpretation is accurate and both have been rejected by more recent historians.
The inaccuracy of this interpretation has been addressed in the answers u/ThucydidesWasAwesome linked. However, I think it's worth addressing what the crusaders themselves thought as well.
To their participants, the crusades, at least those going to Palestine, were defensive. That Jerusalem had been under Muslim rule for over 400 years before the First Crusade was irrelevant. They were reclaiming lands that had once been Christian, lands where Christ had lived and died. As followers of Christ, these lands were theirs by right. Not only was the Holy Land held by non-Christians, but the crusaders had also heard stories that these occupiers had attacked pilgrims to the Holy Land. These stories weren't wholly inaccurate. In 1009 al-Hakim, the Fatimid caliph then ruling Jerusalem, destroyed the Church of the Holy Sepulchre. This was one of Christianity's most holy sites. It supposedly marks Calvary, where Jesus was crucified, and also contains his empty tomb. The Holy Sepulchre was eventually allowed to be rebuilt, but even at its completion in the mid-eleventh century, half of the original church was still in ruins. The crusades were also seen at the time as defending Eastern Christians, members of the various churches based in the Levant like the Syriac and Marionite Christians. The original text of Urban II's call for the First Crusade at Clermont in 1095 doesn't survive, but four different versions of it were recorded. One of these, Baldric of Dol's, has Urban admonitioning Latin Christians for fighting amongst themselves and not helping their fellow Christians in the East, saying 'It is much less evil to brandish the sword against the Muslims; in a particular case it is good, because it is charity to lay down lives for friends.'
This view of the crusades as acts of Christian love and charity was also strong. When Urban II preached the crusade in Italy, he praised those crusaders that had given themselves to the expedition 'out of love of God and their neighbour'. Successive popes and ecclesiastics echoed these feelings. Sewing a cross onto their clothes, the crusaders would be more than aware of the following biblical passage: 'And whosoever doth not bear his cross, and come after me, cannot be my disciple.' (Luke 14, 27). This passage was alluded to in calls for crusades. A bull of Celestine II in 1144 described the Templars as 'renouncing earthly desires and possessions, bearing his cross, are followers of Christ'.
To a man embarking on the First Crusade in 1096 then, this was a defensive expedition aiming to recover and protect Christianity's most holy sites, an act of Christian charity following Christ's own commandments, and an attempt to defend the Eastern Christians.
Sources:
Alfred J. Andrea and Andrew Holt, Seven Myths of the Crusades (2015). Conor Kostick, The Crusades and the Near East: Cultural Histories (2011). Jonathan Riley-Smith, 'Crusading as an act of love', History, Vol. 65, No. 214 (June 1980), pp. 177-92.