  Using the N-body+Smoothed particle hydrodynamics code, ChaNGa, we identify
two merger-driven processes\textemdash disk disruption and supermassive black
hole (SMBH) feedback\textemdash which work together to quench L$^*$ galaxies
for over 7 Gyr. Specifically, we examine the cessation of star formation in a
simulated Milky Way (MW) analog, driven by an interaction with two minor
satellites. Both interactions occur within $\sim$100 Myr of each other, and the
satellites both have masses 5 to 20 times smaller than that of their MW-like
host galaxy. Using the genetic modification process of \cite{Roth2016}, we
generate a set of four zoom-in, MW-mass galaxies all of which exhibit unique
star formation histories due to small changes to their assembly histories. In
two of these four cases, the galaxy is quenched by $z = 1$. Because these are
controlled modifications, we are able to isolate the effects of two
closely-spaced minor merger events, the relative timing of which determines
whether the MW-mass main galaxy quenches. This one-two punch works to: 1. fuel
the primary halo's supermassive black hole (SMBH) at its peak accretion rate;
and 2. disrupt the cold, gaseous disk of the host galaxy. The end result is
that feedback from the SMBH thoroughly and abruptly ends the galaxy's star
formation by $z\approx1$. We search for and find a similar quenching event in
{\sc Romulus25}, a hydrodynamical $(25\,\mathrm{Mpc})^3$ volume simulation,
demonstrating that the mechanism is common enough to occur even in a small
sample of MW-mass quenched galaxies at $z=0$.

Astrophysics of Galaxies