Jihad.

One of the worlds most popular religions is Islam. Founded by Muhammad around 620 A.D., Islam contains ideas, rules and concepts different than most. These concepts include the hajj, the Hegira and jihad.
Jihad is literally defined as "struggle in the way of God". This word can sometimes be translated as "holy war". This principle basically says that Muslims are to fight against all people who use idolatry and do not worship Allah. Daniel Pipes wrote an article on jihad for New York Post saying, "It means the legal, compulsory, communal effort to expand the territories ruled by Muslims at the expense of territories ruled by non-Muslims." (source)
The jihad grew out of the Arabic tradition of tribal rampages. Islamic followers would raid certain regions and countries in the name of Allah. These raids went on for over 100 years. It was finally stopped n Tours, France. They would raid these regions attempting convert anyone from another religion to convert to the Islamic religion.
Most Muslims believe that jihad is an everlasting war. They believe that it never ends. They may not be physically fighting for their religion, but they in a religious war to convert other religions to worshiping Allah. The jihad will continue until the end of existence, they believe.
Islam, being the number one religion in the world, is mainly this widespread because of jihad. They forced their ways and converted thousands upon thousands of people to the way of Allah. The jihad is not over, and I personally believe that as time goes on, the Islamic will only get pushed even more and become more widespread through the world.

The Just War Theory

"True religion looks upon peaceful wars that are waged,
not for the motives of anggrandisement, or cruelty,
but with the object of securing peace, punishing those who do evil,
and supporting the good." Saint Augustine. (source)

Saint Augustine, referred to as the theologian of war, really pushed the Just War theory in his time. The principle of Just War basically deals with justifying why we fight wars and the way we fight, or should fight, them.
Saint Augustine was given a false title. In reality, he should be called the theologian of peace. He was not unfamiliar with violence, but he mourned the cruelty of war. He wrote letters against such cruelty. He hoped that this principle of justice would be adopted. One time, he even wrote, "it is better to kill war with words than human beings with swords." (source)
This theory basically says that war is fair and just when executed with right intentions, a just cause, and based on the military of both. All of these requirements are listed in the Jus ad bellum of the Just War theory. Once a war has begun, this theory also outlines how it shoud play out. These requirements are stated in what is called the Jus in bello. Last is the end of the war. There must a be a just cause to why the war is ending and good intentions in ending the war are required. Even truth and reconciliation can play a role in the end of the war. The full list is called the Jus post bellum.
Just War is pretty much the keeping of peace through controlled and fair fighting. Without a principle like this, wars would have destroyed us a long time ago. Civilizations would have been crushed. Without Just War, our world would be chaos.