(Edit: ugh, this turned into a Russian novel. Sorry about that.)
Well, first of all I'm sure you agree that a "nation" as a set of ruling institutions doesn't have any human characteristics at all, it's just a noun, and that's what I was talking about. "The US" as a territorial and political entity isn't animate and doesn't have any animate characteristics. American principles have normative characteristics -- the freedoms that citizenship is supposed to guarantee are good things, for example. Other characteristics may not be so good, though on the whole there aren't a lot of other sets of rules I'd prefer to live under. So patriotism makes sense both as a way of saying "I approve of and will defend these freedoms and rights" and also as the next concentric circle of allegiance after family and whatever locality (congregation, hockey team, etc) turns you on.
But let's talk about a "nation" as its people. Certainly the British people showed amazing courage during WW2, as did the Russian people. But of course the French people showed amazing courage during WW1, when they lost some insane number like 20% of their male population of age and still didn't buckle, and that ruins a lot of great French jokes.
What did all those cases have in common? A populace was directly attacked and so of course our hearts go out to them, and every act of kindness or stiff upper lippedness is magnified because when we put ourselves in their shoes we imagine the suffering. Add to this that people did all sorts of amazing individual acts putting themselves at risk so that others could be safe, and we marvel at the bravery, in the same way we do when somebody runs into a burning house or dives into freezing water to save somebody.
Put it another way. If we are going to claim countries are "brave" or "cowardly," what are we? The troops who risk their lives on the front lines we call "brave" and the vast majority are, particularly since this is an all-volunteer military. But is the guy who pilots drones from his cubical in Nevada brave? Doesn't what we call "bravery" shade off into an emotional statement of approval or disapproval? The guys who flew an airliner in the WTC were insane and immoral and wrong and ef them, but it took stones. Would any of us call that "bravery," or does "bravery" mean "I respect what you're doing" and not "you faced down fear for what you believed in"?
I know many people who served in WW2 and when they talk about it they talk about being either bored or terrified most of the time, relying on what little training they had, and doing what they had to both to survive and to help the man next to them survive because he seemed like an OK guy who would do the same for him. If that's what courage is, and I think it is, then courage is about being human rather than British or American. London in 1940 displayed a huge number of brave people, and part of that was people hanging together rather than saying "I'm looking out for #1, the rest of you are on your own." The very ethic, in fact, that many people sneer at when it's displayed in peacetime.