  This article reports a measurement of the proper motion of Leo II, a dwarf
galaxy that is a likely satellite of the Milky Way, based on imaging with the
Hubble Space Telescope and Wide Field Camera 3. The measurement uses compact
background galaxies as standards of rest in both channels of the camera for two
distinct pointings of the telescope and a QSO in one channel for each pointing,
resulting in the weighted average of six measurements. The measured proper
motion in the the equatorial coordinate system is (mu_alpha, mu_delta) = (-6.9
+- 3.7, -8.7 +- 3.9) mas/century and in the Galactic coordinate system is
(mu_l, mu_b) = (6.2 +- 3.9, -9.2 +- 3.7) mas/century. The implied space
velocity with respect to the Galactic center is (Pi, Theta, Z) = (-37 +- 38,
117 +- 43, 40 +- 16) km/s or, expressed in Galactocentric radial and tangential
components, (V_r, V_tan) = (21.9 +- 1.5, 127 +- 42) km/s. The space velocity
implies that the instantaneous orbital inclination is 68 degrees, with a 95%
confidence interval of (66 deg, 80 deg). The measured motion supports the
hypothesis that Leo II, Leo IV, Leo V, Crater 2, and the globular cluster
Crater fell into the Milky Way as a group.
