Principle number 19 of Basic Elements of Islam: Islam provides a huge and inspiring goal. Leaders of countries, companies, and religions have all discovered that you can get the most motivation and enthusiasm from your followers if you provide them with an expansive vision — an enormous goal. In the Islamic idea-collection, the goal calls for a continuous effort to expand the domain of Islamic law until the entire world is subjugated to it.
Many religions have the goal of converting others to the faith, but Islam has a method available nobody else has: To expand by seizing and converting governments to Sharia, or using the method of gaining one small, incremental concession or accomodation after another until Sharia law is being followed.
Once the whole world is following Islamic law, peace will reign. That's why even terrorists can say with complete sincerity, "Islam is a religion of peace."
The Quran says it's better if non-believers accept Islam and become Muslims without force. But if they refuse, then you must do what you can to at least get them to live by the laws of Allah. Read more about this.
So Muslims have been given quite a mission: To create a one-world government. An Islamic world. World peace. It is an enormous and inspiring and motivating goal, and creates a strong unity of purpose.