Nationalism is the belief in a common culture, or people, and its connection to a particular country. For a deeper discussion of the importance of nationalism(s), see below.
For Deeper Discussion
Countries on the Cusp—The Power of Nationalism (by Martha Legace) This will take you to an interview with Harvard Business School Professor Rawi Abdelal where he describes the power nationalism has over new countries—and their far-reaching effects.
Another perspective on nationalism is discussed in the following article by Jerry Z. Muller in Foreign Affairs (March/April 2008), titled Us and Them: The Enduring Power of Ethnic Nationalism.