Understanding coronal mass ejection (CME) energetics and dynamics has been a long-standing problem, and although previous observational estimates have been made, such studies have been hindered by large uncertainties in CME mass. Here, the two vantage points of the Solar Terrestrial Relations Observatory (STEREO) COR1 and COR2 coronagraphs were used to accurately estimate the mass of the 2008 December 12 CME. Acceleration estimates derived from the position of the CME front in three dimensions were combined with the mass estimates to calculate the magnitude of the kinetic energy and driving force at different stages of the CME evolution. The CME asymptotically approaches a mass of 3.4 ± 1.0 × 1015 g beyond ~10 R ☉. The kinetic energy shows an initial rise toward 6.3 ± 3.7 × 1029 erg at ~3 R ☉, beyond which it rises steadily to 4.2 ± 2.5 × 1030 erg at ~18 R ☉. The dynamics are described by an early phase of strong acceleration, dominated by a force of peak magnitude of 3.4 ± 2.2 × 1014 N at ~3 R ☉, after which a force of 3.8 ± 5.4 × 1013 N takes effect between ~7 and 18 R ☉. These results are consistent with magnetic (Lorentz) forces acting at heliocentric distances of lsim7 R ☉, while solar wind drag forces dominate at larger distances (gsim7 R ☉).