Patriotism is good if it is mild. It serves to create and maintain socio political cohesion and stability, and hence generates strength within a nation. At this level, so long as it does not become exclusionist and separarist (both of whcih invariably lead to strife), it can coexist with internationalism and "global" unity, and is hence a positive think.
Jingoism and national extremism, on the other hand, is one of the greatest evils in the world's history. It becomes extremist and separatist, promotes nationalistic supremacy, and becoems an anathema to international relations and diplomacy - which, of course, must invariably lead to nations being at daggers drawn, hence resulting in conflict. At this level it is also dangerous, as it becomes a tool for manipulating and exhorting people (the masses, if you will) to fight and follow without question the orders, demands and "truths" of the government in question (e.g. wwII Germany, modern day China). Important truths are often ignored and negated, and people often refuse to accept the wrongs that their nation has comitted now and in the past (e.g. the fact that many Chinese students don't believe Tianamen square incident occuered; cf with the misuse of history). The people become blind, believing that the country is a perfect (both logically/functionally and morally) utopia simply because it is "their wonderful country .
