system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The Crusades in the Middle East way back.

I'm not too sure if this thread will be picked up, but seeing as how I'm currently playing the first Assassin's Creed, I would love to see how the game ties in with real-life history. I don't know much about what happened - only that Spain/England/Britain/Europe in general (?) invaded the Middle East to reclaim what they thought should be their land, in the "name of God". Other than that, I don't know much at all about it.
gpt
The Holy Land--what's now Israel / Palestine, or the site of most of the events of the Old and New Testaments of the Bible--had for centuries been ruled by the Byzantine Empire. That name comes from its capital at Byzantium, also known as Constantinople and present-day Istanbul.

The Byzantine Empire was a Christian empire, though the kind of Christianity it followed (Orthodox Christianity) was different from the kind of Christianity (Roman Catholicism) followed in France, Germany, England, and the rest of Western Europe. 

But by the year 1000 the Byzantine Empire was in big trouble. It kept losing territory to Muslim invaders from the east. When it lost control of the Holy Land to Muslim Turks, that got lots of people in Western Europe alarmed. 

In 1095, the Byzantine Empire asked Pope Urban II, head of the Catholic Church for help. Urban proclaimed a crusade, and encouraged people from Western Europe to head to the Holy Land to get it back from the Muslims. For the next 200 years, crusaders would make a number of attempts to try to win the area back, but could only hold the land for a short time.

So the crusaders didn't see themselves as invading the Holy Land. They saw themselves as helping to kick out Muslim invaders.