It's comparable to saying you love your family. It doesn't mean that you think your family is inherently better than other families; it simply means that you recognize your family has value to you in a way that others may not share. A love of one's nation comes from one's brain and shared experiences, while nationalism comes from irrational passion and a desire to out "the other." As was the case with the first Crusade and with World War I, we can expect this tendency to expand as social tensions rise and economies collapse.
WWI Anti-German Propgaganda by US Government
This, in conjunction with a series of complicated military alliances, runaway imperialism, and an emphasis on aggressive military tactics, lead to the largest war mankind had ever seen.
If nationalism truly begins to take hold in the US, the possibility of a larger war will increase dramatically. Geopolitical and economic challenges may be pushing nations to war, but nationalism can accelerate the process far more rapidly than most realize.