  We study dynamical friction in interacting relativistic systems with
arbitrary mean free paths and medium constituent masses. Our novel framework
recovers the known limits of ideal gas and ideal fluid when the mean free path
goes to infinity or zero, respectively, and allows for a smooth interpolation
between these limits. We find that in an infinite system the drag force can be
expressed as a sum of ideal-gas-like and ideal-fluid-like contributions leading
to a finite friction even at subsonic velocities. This simple picture receives
corrections in any finite system and the corrections become especially
significant for a projectile moving at a velocity $v$ close to the speed of
sound $v\approx c_s$. These corrections smoothen the ideal fluid discontinuity
around the speed of sound and render the drag force a continuous function of
velocity. We show that these corrections can be computed to a good
approximation within effective theory of viscous fluid dynamics.
