  The spin of dark halos has been shown to significantly affect bar formation
and evolution in disk galaxies. To understand the physical role of the halo
spin on bar formation, we run $N$-body simulations of isolated, Milky Way-sized
galaxies by varying the halo spin parameter in the range $-0.16 \leq \lambda
\leq 0.16$ and the bulge mass. We find that our adopted halo \emph{alone} is
subject to swing amplification of an $m=2$ non-axisymmetric mode rotating in
the same sense as the halo, which assists or inhibits the bar formation in a
disk depending on its sense of rotation. The $m=2$ mode in the disk, growing
via swing amplification, interacts constructively (destructively) with the
$m=2$ mode in the prograde (retrograde) halo, promoting (delaying) bar
formation. A bar grows by losing its angular momentum primarily to a halo.
Since the halo particles inside (outside) the corotation resonance with the bar
can emit (absorb) angular momentum to (from) the bar, the bar pattern speed
decays slower for larger $\lambda>0$, while it decreases relatively fast almost
independent of $\lambda\leq0$. Models with a strong bar develop a boxy
peanut-shaped bulge. In models without a bulge, this occurs rapidly via
buckling instability, while the bars with a bulge thicken gradually without
undergoing buckling instability. Among the models considered in the present
work, the bar in the $\lambda = 0.06$ model with a bulge of 10\% of the disk
mass best describes the Milky Way in terms of the bar length and pattern speed.
