  Monte-Carlo simulations in conjunction with finite-size scaling analysis are
used to investigate the $(H,T)$-phase diagram in uniaxial anisotropic high-
$T_c$ superconductors, both in zero magnetic field and in intermediate magnetic
fields for various mass-anisotropies. The model we consider is the uniformly
frustrated anisotropic Villain Model. In zero magnetic field, and for all
anisotropies considered, we find one single second order phase transition,
mediated by an Onsager vortex-loop blowout. This is the superconductor-normal
metal transition.A comparison with numerical simulations and a critical scaling
analysis of the zero-field loop-transition yields the same exponent of the loop
distribution function at the critical point. In the intermediate magnetic field
regime, we find two anomalies in the specific heat. The first anomaly at a
temperature $T_m$ is associated with the melting transition of the flux-line
lattice. The second anomaly at a temperature $T_z$ is one where phase coherence
along the field direction is destroyed. We argue that $T_m=T_z$ in the
thermodynamic and continuum limit. Hence, there is no regime where the flux
line lattice melts into a disentangled flux-line liquid. The loss of phase
coherence parallel to the magnetic field in the sample is argued to be due to
the proliferation of closed non-field induced vortex loops on the scale of the
magnetic length in the problem, resulting in flux-line cutting and
recombination. In the flux-line liquid phase, therefore, flux-lines appear no
longer to be well defined entities. A finite-size scaling analysis of the delta
function peak specific heat anomaly at the melting transition is used to
extract the discontinuity of the entropy at the melting transition.This entropy
discontinuity is found to increase rapidly with mass-anisotropy.
