We report new experimental results on the dewetting of a mercury film (A) intercalated between a glass slab and an external nonmiscible liquid phase (B) under conditions of a large equilibrium contact angle. The viscosity of the external phase, eta(B), was varied over 7 orders of magnitude. We observe a transition between two regimes of dewetting at a threshold viscosity of eta(B)* approximate to (rho(A)e vertical bar(S) over tilde vertical bar(1/2), where rho(A) is the mercury density, e is the film thickness, and vertical bar(S) over tilde vertical bar is the effective spreading coefficient. For eta(B) < eta(B)*, the regime is inertial. The velocity of dewetting is constant and ruled by Culick's law, V approximate to (vertical bar<(S)over tilde>vertical bar/(rho(A)e))(1/2). Capillary waves were observed at high dewetting velocities: they are a signature of hydraulic shock. For eta(B) > eta(B)*, the regime is viscous. The dewetting velocity is constant and scales as V approximate to vertical bar(S) over tilde vertical bar/eta(B) in the limit of large eta(B). We interpret this regime by a balance between the surface energy released during dewetting and the viscous dissipation in the surrounding liquid.