Question

How did nationalism affect the foreign affairs of the United States

Answer 1

Nationalism-the loyalty and devotion to one's nation. 
For one, people in America were like "lets go to war with Britain" which spurred the War of 1812. So if that's any correlation to how it affected foreign affairs, there ya go. 
Our foreign affairs were affected because we were proud to be Americans. We got more active in conflicts and defended our country and its people mainly due to agrarianism (You'll probably learn about that in US History in HS), which was the belief Thomas Jefferson had that if people owned land, they would fight for this country. 
To really answer your question, we were on and off friends with our foreign comrades. We went to war as a result, and established our power in the world.