Maybe I phrased that poorly, because you're right that we've engaged in this 'otherization' since the beginning of time (for good reasons and bad). Maybe a more important factor is that we're increasingly putting OURSELVES into boxes, which increases the polarization of discourse (because now WE identify as a group and not a whole, and those OTHER groups aren't US so clearly they're wrong/bad/etc).

This ebbs and flows throughout history (or rather the US and THEM changes, based on circumstances). There are plenty of unifying events in US history that lead to more of an 'American' identity (wars, 9/11, generally dangerous times or shitty events drive unification out of necessity). Those aren't the times we're in now, but it's cyclical. Some day we will be again, then not, ad infinitum. The pendulum always swings.