Looking at maps depicting the world during the 7th and 8th centuries AD, the Islamic Caliphates seem to have expanded to include most of central and western Asia along with Mediterranean North Africa in the span of just 80 years. How was this achieved?
I've written quite a bit about this topic elsewhere, which I hope provides quite a comprehensive view of this period as a whole. The most relevant is my answer on why the Arab conquests succeeded, but other answers on how Christians at the time saw Islam, on whether Islam was seen as a Christian heresy, and on whether Rome and Persia had any warning before the conquests began might be interesting for you as well. /u/textandtrowel's answer on just how Islamic the conquests were is relevant too. There are quite a few book recommendations included in these answers, but I'm happy to talk more about the sources here if it is needed :)