  We present calculations of the ground state and excitations of an anisotropic
dipolar Bose gas in two dimensions, realized by a non-perpendicular
polarization with respect to the system plane. For sufficiently high density an
increase of the polarization angle leads to a density instability of the gas
phase in the direction where the anisotropic interaction is strongest. Using a
dynamic many-body theory, we calculate the dynamic structure function in the
gas phase which shows the anisotropic dispersion of the excitations. We find
that the energy of roton excitations in the strongly interacting direction
decreases with increasing polarization angle and almost vanishes close to the
instability. Exact path integral ground state Monte Carlo simulations show that
this instability is indeed a quantum phase transition to a stripe phase,
characterized by long-range order in the strongly interacting direction.
