  Using general scaling arguments combined with mean-field theory we
investigate the critical ($T \simeq T_c$) and off-critical ($T\ne T_c$)
behavior of the Casimir forces in fluid films of thickness $L$ governed by
dispersion forces and exposed to long-ranged substrate potentials which are
taken to be equal on both sides of the film. We study the resulting effective
force acting on the confining substrates as a function of $T$ and of the
chemical potential $\mu$. We find that the total force is attractive both below
and above $T_c$. If, however, the direct substrate-substrate contribution is
subtracted, the force is repulsive everywhere except near the bulk critical
point $(T_c,\mu_c)$, where critical density fluctuations arise, or except at
low temperatures and $(L/a) (\beta\Delta \mu) =O(1)$, with $\Delta
\mu=\mu-\mu_c <0$ and $a$ the characteristic distance between the molecules of
the fluid, i.e., in the capillary condensation regime. While near the critical
point the maximal amplitude of the attractive force if of order of $L^{-d}$ in
the capillary condensation regime the force is much stronger with maximal
amplitude decaying as $L^{-1}$. Essential deviations from the standard
finite-size scaling behavior are observed within the finite-size critical
region $L/\xi=O(1)$ for films with thicknesses $L \lesssim L_{\rm crit}$, where
$L_{\rm crit}=\xi_0^\pm (16 |s|)^{\nu/\beta}$, with $\nu$ and $\beta$ as the
standard bulk critical exponents and with $s=O(1)$ as the dimensionless
parameter that characterizes the relative strength of the long-ranged tail of
the substrate-fluid over the fluid-fluid interaction. We present the modified
finite-size scaling pertinent for such a case and analyze in detail the
finite-size behavior in this region.
