One of the most powerful of the centripetal forces, based on the concept of allegiance to a single country and the ideals and way of life it represents, is known as:

Nationalism

Explanation:

Nationalism is a way of thinking that says that some groups of humans, such as ethnic groups, should be free to rule themselves. Nationalists think that the best way to make this happen and avoid control or oppression by others is for each group to have their own nation.