  Upon reduction of the film thickness we observe a metal-insulator transition
in epitaxially stabilized, spin-orbit coupled SrIrO$_3$ ultrathin films. By
comparison of the experimental electronic dispersions with density functional
theory at various levels of complexity we identify the leading microscopic
mechanisms, i.e., a dimensionality-induced re-adjustment of octahedral
rotations, magnetism, and electronic correlations. The astonishing resemblance
of the band structure in the two-dimensional limit to that of bulk
Sr$_2$IrO$_4$ opens new avenues to unconventional superconductivity by "clean"
electron doping through electric field gating.
