Yeah, Memorial Day in America from what I know is just a holiday, just an excuse for a long weekend. I wonder if part of it is that in Israel, there's a mandatory military service, so everyone knows people who died in the army, and most people you meet are veterans (I am too, for instance, as are my parents, my siblings, almost all my cousins, uncles, etc. and we're not a "military family", it's just the norm here).

I wonder too if in America, the schism and lack of solidarity leads to the political divide, or whether it's the other way around.

And it's not about patriotism, it's about solidarity. It's a shame that the way most people think of patriotism and nationalism when they hear solidarity, because what that means is, solidarity with some, by presenting ourselves as distinct from others. Showing we belong, by showing those who don't.
And if you follow that path, you stop feeling as if everyone belongs, and slowly, the sense of solidarity erodes.