What is the rhetorical strategy called when the author sets up an "us vs them" situation?

wedge issue

This is a strategy in politics that forces people to take one side or the other. You can't sit on a wedge (unlike a fence). I am not sure if this is exactly what you are after. Maybe:

Creating a divide

Or

Divisive politics

polarization

noun

1. a sharp division, as of a population or group, into opposing factions.
   _{dictionary.reference.com}

Polarization is a strategy used in ideological discourse. See Wikipedia articles on social identity theory and Ingroups and outgroups.

The polarized (Us vs. Them) structures of ideological discourse typically use various forms of rhetorical "figures," such as hyperboles and euphemisms, to emphasize or mitigate assumed in-group and out-group properties.
_{Google Books: Encyclopedia of Philosophy and the Social Sciences}

In logic, this is called a False Dichotomy, and is considered a fallacy. The classic example is "you are either for us, or against us". This phrase has been used by US presidents and Sith Lords. Neutrality is not an option to the speaker who uses this false dichotomy.