Principle number 5 of Basic Elements of Islam: Permission to spread the religion by war. This is another successful innovation (source). Although some other religions have spread themselves using force, they had very little justification from their own religious doctrines to do so.
Not so with Islam. Expanding by conquest is very much accepted and encouraged by the idea-collection. Islamic teachings present it this way: The non-Muslims need to be saved from the sin of following laws other than Allah's. If they won't voluntarily change their laws to Sharia, then it is a Muslim's duty to insist. The world cannot be at peace until every government on earth follows the laws of Allah.
Mohammad's own experience showed the example — an example, says the Quran 91 times, that every Muslim should follow. At first, Mohammad tried to spread Islam by peaceful means. After thirteen years he'd gained 150 converts.
But then he changed tactics and started using caravan raids, warfare, executions, ransoming captives, and assassination, and within ten years he converted tens of thousands. After he died, his followers used the same tactics and converted millions. And by now it is one and a half billion.
The use of warfare combines synergistically and powerfully with the instruction to create an Islamic state. So Islam spread quickly as their armies got bigger. They conquered and set up Islamic states, most of which have lasted to this day, and the laws within an Islamic state make Islam very difficult to dislodge. The laws also make it very advantageous to convert to Islam.
This is one of the most effective methods ever invented for getting an idea-collection followed by huge numbers of people. It's a method of control and indoctrination similar to those used successfully in communist and totalitarian states. But as you'll discover below, Islam makes unique use of the power of the law to enforce complete conversion to the religion.
Islam started under unique conditions. All other major religions were started within an already-existing state. Islam is an historical exception to this rule.
Any organized government will, of course, put a stop to violent uprisings of a rebellious political group — especially one that wants to wage war and apply its own system of law. Christianity arose within the Roman Empire, for example. If Christianity had been a militant or political uprising, Rome would probably have killed or imprisoned the followers. Probably many military or political religions did start up then, but we've never heard of them. They couldn't get off the ground.
But Islam arose in Arabia when there was no central ruling power. The whole area was comprised of individual tribes. Under those circumstances, conversion by war and the use of force was possible.