I believe that you have bought into the re-writing of history.

Jerusalem, as well as Constantinople (today's Istanbul), were Christian enclaves. Heck, Constantinople was the capital of the Eastern Roman Empire. Moreover, there was a "king" of Jerusalem.

Muslims invaded, not the other way around.

Europe was besieged by Muslim raids and invasions. This is not even dealing with the time of the "Crusades". Probably, one of the largest defeats of the muslims occurred when over 300,000 muslims were defeated by fewer than 40,000 at the siege of Vienna by the Holy League of Christian States. These attacks occurred even though there was a signed peace treaty between Europe and the muslim hordes.

Even into the 20th Century, muslims continued to invade Europe (look at the terrible atrocities that the Ottomans committed well into the early 20th Century).

Rather than Christians attacking muslims, it was Christians simply defending themselves from muslim invasions.