In fact, Muslims attacking Israel, a civilised country, for free, should be more than enough reason for any civilised human bieng to consider that Islam is at war with all of us. But many liberals , including leftist "peaceniks" Jews, think that it is an issue of Jews only, and Gentiles are uncalled to fight Muslims.
Also, the fact that Muslims clearly state they want to conquer the world is another valid reason, but leftists would always claim that "the world" is a very vague and unespecific term, and that there are many other cults which have similar mystical prophesies which never attempt to make real. (They fail to realise that, while other cults and ancients pagans might have had some ideas about owning the world, the world only meant their region to them, or maybe they only expected some idol to give the them the world, but never had made any military or terrorist attempts to conquer it).
Regarding that "prophesy", it must be in the Hadiths (which most Muslims consider not holy script but binding anyway), and it's pretty silly of them to claim that it refers to strengthening Christian rule. In fact, Islam at its beginnings, sometimes, considered a place conquered from pagans if it was under the rule of a monotheistic religion, but there were no pagans in Rome in the VII century, neither were there pagans in Constantinople. How can they claim the "prophesy" refered to Christian rule in Rome, something which happened 300 years before Islam? That's a silly way to hide their true intentions!