In 624, Islamic Prophet Mohammed led a raid for booty and plunder against a Meccan caravan, killing 70 Meccans for material gain. From that day on, Mohammed (who died in 632) and his followers continued with invasions and conquests, eventually conquering the bulk of western Arabia and southern Palestine. These acts of aggression, subjugation, and imperialism went on unchecked by the west until the response issued by Pope Urban II in 1095 - which started the Crusades.
Christians were ultimately unsuccessful at stopping Muslim expansion into the west. The Ottoman Empire invaded and colonized Greece, the Balkans, Romania, Bessarabia and Hungary; they stopped only at the outskirts of Vienna, Austria in 1529.
In these conflicts, Christians didn't always act in accord with the teachings of Jesus Christ, but it's difficult to substantiate the claim of unilateral aggression on the side of Christians during the Crusades.