How have tribes caused problems in the history of Islam?

The Islamic tribes have always caused conflict or problems in the religion of Islam. From the times of Muhammad to recent issues with the Sunnis and Shiites, the division of tribes and the people of one religion has been a conflict in itself. Obviously, any break up of a common people could result in problems. This is because they feel the need to be different, in order to stand out, and the other tribes or sections would see them as contrasting with the norm and their reactions would undeniably be filled with hatred. Problems like this could happen in any situation; not just tribes or religions. It happens all the time today, even in high school. For example, if all girls in our grade were friends but a few girls in that group decided  that they all like chocolate ice cream, while the others liked vanilla, and left the vanilla-loving girls to be by themselves, the pro-vanilla girls would become angry that people had left their group and problems and drama would start. Even though this is a very modern and less intense version of what happens with tribes in any type of group or situation, it still demonstrates the key factors of how problems can start if people begin to differ on very little subjects. Another huge issue that was brought to Islam because of tribes was that Muhammad was born into a very poor tribe, so he had a very tough time trying to win his way to the top. Because the poor had barely any say in society, Muhammad had to work harder than any other to reach his goal in trying to change Islam. Today, we see the conflict between the Sunnis and Shiites going on daily, just because of a couple differences in the way they see Islam and the world. They're all one people, can't they just all try to get along?