If you define true anarchy as 100% "every person for themselves," it's not sustainable because people will always form groups (government, basically) for protection and for "justice" from other people out to dominate and destroy.

Russia invade the US? Um, no. Believe it or not, not every country in the world follows the western model of "exceptionalist" imperialist conquest in order to prop up an otherwise unsustainable debt-based fantasy-casino economy (and no, Crimea is very very clearly not such a case).