The only thing that bugs me is when I watch a USA game and people, who I assume live here, are wearing and cheering for the foreign country we are playing. cough-cough Mexico.
My wife, kids, and me went to Croatia vs Mexico at LA Colosseum a few years back. 80k Mexico supporters vs 5k Croatian supporters. The Mexicans were extremely well behaved and actually took matters into their own hands when the usual minority of village idiots threw water bottles at our packed ranks.
And there were quite a few American flags. Croatian-Americans almost always have an American flag feature on our Croatia t-shirts. Croatians in Anglo-American nations support them to the nth degree.
Funny, because Croatians in Germany and France aren't nearly as supportive if those countries.