just kill em all
This is false as a matter of history. Islam broke up internally and did so relatively early, because as a single civilization it had made no stable arrangements for political rule, succession, and loyalty of a military to the political ruler. Later Islamic states did continue to expand by conquest where they could, and Islam as a whole also spread by conversions without conquests, especially south into Africa and east through Asia, both central and south. The Crusades occurred after the internal break up on the one hand, and did not halt the expansions on the other.
Externally and militarily, Islam was checked in the west at the battle of Tours in the 8th century AD. Advances on Vienna by the Turks - only one polity within Islam but a dominant one later on, in the middle east and southeast Europe - were not definitively turned back until the 18th century. They had not conquered Constantinople until the 15th century.
But the main issue in Islamic political history was internal break up. The first dynasty after the death of Muhammad was based in Syria and run by an Arab nobility enriched by the early conquests. It did not last because this aristocracy grew soft, ceasing to be military in nature, with more of the rigors of army duty falling to Persians (from what is north-eastern Iran today). A balancing act was attempted between Arab and Persians in the army. Eventually Turkic mercenaries and then slaves were brought in from central Asia in an effort to keep the Persian faction from dominating the army. The dynasty shifted to what is now Iraq, half-way between the Syrian and Persian heartlands. But the Turks proved uncontrollable in turn, and were soon setting up rulers on their own account, much like what happened with the Praetorian guard attempt in the case of Rome. Their puppets were not recognized by the western provinces, which broke away - Moorish north Africa and Fatamid Egypt. Islam was splintered into half a dozen states at the time of the first crusade.
The crusades produced an attempt to reunite portions of the Islamic world, which was only partially successful. Saladin, from a base in Syria, conquered Egypt and reunited much of what today we call the middle east. But never controlled north Africa or Spain (then Islamic of course). This base of power proved sufficient to repulse the crusaders - eventually. But the whole Islamic world east of Africa was then smashed to kindling by the Mongols, who remained animist at the time. In the aftermath of Ghengis, many of the successors converted to Islam and then fought among themselves for the spoils of his former empire. Tamerlane reunited the western portion of Ghengis' conquests, laying much of them waste in the process. After his death, the Moguls (a later Mongol-successor faction and Islamicized) took over the previous position in northern India (today Pakistan), while the Turks took over the western portion. The Turks continued to fight westward, the whole previous Islamic world ahead of them dissolving into conflicting warlords - except for Moorish Spain and north Africa.
In was in late medieval or early modern times that the "reconquista" drove the Moors from Spain, while at the same time the Turks drove the Byzantine Greeks from Asia Minor. Eventually the Turks took Constantinople - the last of the eastern Roman empire - and Castille and Aragon took Grenada, driving the Moors from Spain - both in the 15th century. That left the pattern that basically lasted down to the 19th century, with the Ottoman Empire (Turkey) controlling all of what today we'd call the Middle East, from Egypt to Iraq. The Turks pressed to the gates of Vienna more than once, in their long rivalry with Austria for control of the Balkans.
Then in the 19th century France took northwest Africa, Britain took Egypt and later Sudan to secure the Suez canal, important to their holdings in India. For a long time British policy supported the Ottomans to avoid Russian aggrandizement, notably in the Crimean war in the middle of the century. In WW I, however, the Ottomans backed the central powers and Britain took the occasion to lead an Arab revolt against Turkish dominion. As a result of which, the UK disposed of the Middle East effectively as it pleased in the aftermath of the war. France received Syria and Lebanon, and Britain allowed the Jews to settle in Palestine. The rest they gave to Arab kings of the house of Faisal (originally Syria too, but the French backed out and deposed that one).
This is hardly a picture of "only stopped by overwhelming force", or any unified monolith, or of a civilization that never brought in outsiders to settle internal power struggles. The central Asians were such outsiders originally. The British certainly were in the rivalry between Arab and Turk. They fought at least as many wars within as without.
Learn a little history before indulging such sweeping generalizations. You will discover what your own experience of mankind ought to tell you anyway, that it is always messier than that, and the natural tendency of most power blocks is to break apart for internal reasons, as soon as external movements, either way, slow down.