  The evolution of a stellar bar transforms not only the galactic disk, but
also the host dark matter halo. We present high resolution, fully
self-consistent N-body simulations that clearly demonstrate that dark matter
halo central density cusps flatten as the bar torques the halo. This effect is
independent of the bar formation mode and occurs even for rather short bars.
The halo and bar evolution is mediated by resonant interactions between orbits
in the halo and the bar pattern speed, as predicted by linear Hamiltonian
perturbation theory. The bar lengthens and slows as it loses angular momentum,
a process that occurs even in rather warm disks. We demonstrate that the bar
and halo response can be critically underestimated for experiments that are
unable to resolve the relevant resonant dynamics; this occurs when the phase
space in the resonant region is under sampled or plagued by noise.
