The nonlinear instability of anti-de Sitter spacetime has recently been established with the striking result that generic initial data collapse to form black holes. This outcome suggests that confined matter might generically collapse, and that collapse could only be halted-at most-by nonlinear bound states. Here, we provide evidence that such a 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, or "oscillatons." 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.