  The nonlinear instability of anti-de Sitter spacetime has recently been
established with the striking result that generic initial data collapses to
form black holes. This outcome suggests that confined matter generically
collapses, and that collapse can only be halted -- at most -- by nonlinear
bound states. Here we provide evidence that such mechanism can operate even in
asymptotically flat spacetimes, by studying the evolution of the
Einstein-Klein-Gordon system for a self-interacting scalar field. We show that
(i) configurations which do not collapse promptly can do so after successive
reflections off the potential barrier, but (ii) that at intermediate amplitudes
and Compton wavelengths, collapse to black holes is replaced by the appearance
of oscillating soliton stars. Finally, (iii) for very small initial amplitudes,
the field disperses away in a manner consistent with power-law tails of massive
fields. Minkowski is stable against gravitational collapse. Our results provide
one further piece to the rich phenomenology of gravitational collapse and show
the important interplay between bound states, blueshift, dissipation and
confinement effects.
