  The formation of our Milky Way can be parsed qualitatively into different
phases that resulted in its structurally different stellar populations: the
halo and the disk components. Revealing a quantitative overall picture of the
Galactic assembly awaits a large sample of stars with very precise ages. Here
we report an analysis of such a sample using subgiant stars. We find that the
stellar age-metallicity distribution p(age, metallicity) splits into two almost
disjoint parts, separated at 8 Gyr. The younger reflecting a late phase of
quiescent Galactic disk formation with manifest evidence for stellar radial
orbit migration; the other reflecting the earlier phase, when the stellar halo
and the old alpha-process-enhanced (thick) disk formed. Our results indicate
that the formation of the Galactic old (thick) disk started 13 Gyr ago, only
0.8 Gyr after the Big Bang, and two Gigayears earlier than the final assembly
of the inner Galactic halo. Most of these stars formed 11 Gyr ago, when the
Gaia-Sausage-Enceladus satellite merged with our Galaxy. Over the next 5--6
Gyr, the Galaxy experienced continuous chemical element enrichment, ultimately
by a factor of 10, while the star-forming gas managed to stay well-mixed.
