  We study proper, isometric actions of nonsolvable discrete groups Gamma on
the 3-dimensional Minkowski space R^{2,1} as limits of actions on the
3-dimensional anti-de Sitter space AdS^3. To each such action is associated a
deformation of a hyperbolic surface group Gamma_0 inside O(2,1). When Gamma_0
is convex cocompact, we prove that Gamma acts properly on R^{2,1} if and only
if this group-level deformation is realized by a deformation of the quotient
surface that everywhere contracts distances at a uniform rate. We give two
applications in this case. (1) Tameness: A complete flat spacetime is
homeomorphic to the interior of a compact manifold. (2) Geometric degeneration:
A complete flat spacetime is the rescaled limit of collapsing AdS spacetimes.
