Years ago in my undergraduate studies at St. Mary's University, one of my political science professors taught a year-long class that called for the class to set up a fictional land's government. We had elections for different offices and each class period we were given situations that this fictional nation faced. As a class we had to follow the laws we set up and come to some sort of way forward.
It was my favorite class.
It was in this class that I was voted as the leader of the opposition party and the debates were often intense. As the opposition leader, I constantly feuded with the the majority ruling party's president. At the end of one intense discussion, our professor pulled the class together for a review of the "legislative activity" and set us up for the next day's events. It was in this review that our professor stated something that has stuck with me to this day.
The danger of patriotism is that it does not allow repentance of the sin of the nation it celebrates.
My professor said this idea was from Dietrich Bonhoeffer, and a simple Google search has pointed to Bonhoeffer's Ethics book as the source for this thought.
The greatness of a nation is in its ability to admit where it has gone wrong, how it is perpetuating sin, atoning for acts of injustice and reconciling with its failures.
When we are unable to admit that our nation has and is participating in sin, then we have fallen prey to the danger of patriotism.