  The physical mechanisms responsible for bar formation and destruction in
galaxies remain a subject of debate. While we have gained valuable insight into
how bars form and evolve from isolated idealized simulations, in the
cosmological domain, galactic bars evolve in complex environments with mergers,
gas accretion events, in presence of turbulent Inter Stellar Medium (ISM) with
multiple star formation episodes, in addition to coupling to their host
galaxies' dark matter halos. We investigate bar formation in 13 Milky Way-mass
galaxies from the FIRE-2 (Feedback in Realistic Environments) cosmological
zoom-in simulations. 8 of the 13 simulated galaxies form bars at some point
during their history: three from tidal interactions and five from internal
evolution of the disk. The bars in FIRE-2 are generally shorter than the
corotation radius (mean bar radius $\sim 1.53$ kpc), have a wide range of
pattern speeds (36--97 km s$^{-1}$kpc$^{-1}$), and live for a wide range of
dynamical times (2--160 bar rotations). We find that bar formation in FIRE-2
galaxies is influenced by satellite interactions and the stellar-to-dark matter
mass ratio in the inner galaxy, but neither is a sufficient condition for bar
formation. Bar formation is more likely to occur, and the bars formed are
stronger and longer-lived, if the disks are kinematically cold; galaxies with
high central gas fractions and/or vigorous star formation, on the other hand,
tend to form weaker bars. In the case of the FIRE-2 galaxies these properties
combine to produce ellipsoidal bars with strengths $A_2/A_0 \sim$ 0.1--0.2.
