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.
> Maybe a more important factor is that we're increasingly putting OURSELVES into boxes
Tribal identity is also not new. If there is anything new, it is the existence of a situation where identity tribes can be maintained while being geographically diffuse and intermixed with other identity tribes, which makes it less likely for conflict between identity groups to be quickly resolved to a more stable state by the losing group being excluded from a geographically region with residual members being expelled, forcibly converted or voluntarily assimilated into the locally dominant group, or killed.
Arguably, this is in many ways an improvement, though it does make ongoing inter-group invective worse.
Interesting thought - maybe, there is a cure. Bear with me now, but isn't the main problem here the fact that massive ego builds a wall between you and other opinions? If so, there is a whole group of substances that do exactly the opposite - promote closeness to peers and temporary suppress ego.
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.