  Spatially resolved ROSAT X-ray and ground-based optical data for the
southwestern region of the Cygnus Loop SNR reveal in unprecedented detail the
very early stages of a blast wave interaction with an isolated interstellar
cloud. Numerous internal cloud shock fronts near the upstream flow and along
the cloud edges are visible optically as sharp filaments of enhanced H-alpha
emission. Faint X-ray emission is seen along a line of Balmer-dominated shock
filaments north and south of the cloud with an estimated X-ray gas temperature
of 1.2 x 10^6 K (0.11 keV) corresponding to a shock velocity of 290 km s^-1.
The main cloud body itself exhibits little or no X-ray flux. Instead, X-ray
emission is confined along the northern and southernmost cloud edges, with the
emission brightest in the downstream regions farthest from the shock front's
current position. We estimate an interaction age of around 1200 yr based on the
observed shock/cloud morphology. Overall, the optical and X-ray properties of
this shocked ISM cloud show many of the principal features predicted for a
young SNR shock -- ISM cloud interaction. In particular, one sees shocklet
formation and diffraction inside the inhomogenous cloud along with partial main
blast wave engulfment. However, several significant differences from model
predictions are also present including no evidence for turbulence along cloud
edges, diffuse rather than filamentary [O III] emission within the main body of
the cloud, unusually strong downstream [S II] emission in the postshock cloud
regions, and confinement of X-ray emission to the cloud's outer boundaries.
