Ever since the advent of national armies at the beginning of the 19th century, serving countries which sooner or later gave equal rights to their Jewish citizens, it has been interesting to see how the Jews relate to service in their armies. Germany's Jews were very proud to serve in WWI. Soviet Jews served en masse - like everyone else - in WWII; I've seen estimates that as many as 200,000 Jews died as soldiers of the Red Army (and they're not counted as part of the six million Jews murdered by the Nazis).
Nowadays, it's a rare European Jew who signs up to the military of his country, even in countries that nominally have conscription.
How about the US? Of course, during WWII Jews were conscripted and fought proudly. Anecdotal evidence indicates that by the 1960s and Vietnam, however, they were preferring not to. Offhand, I can think of only one single American Jewish acquaintance of my generation who has served in the US military. Here's an article from the Forward about Captain Benjamin Sklaver, who was killed recently in Afghanistan; while it lacks full statistics, it seems to say there aren't many Jews in the American military.
Seen from the perspective of an Israeli, this is a bit strange: America is at war, after all, and it's very (very) good to its Jews, so you'd think more of them might wish to pay back a civic debt. But perhaps not.