A route to break molecular nitrogen (N2) under mild conditions is demonstrated by N2 gas cracking on, and incorporation into, lanthanide films. Successful growth of lanthanide nitride thin films, made by evaporation of lanthanides in a partial N2 atmosphere at room temperature and pressure as low as 10-4 Torr, is confirmed using X-ray diffraction. In situ conductance measurements of pure lanthanide thin films exposed to N2 gas show an immediate surface reaction and a slower bulk reaction. Finally, we report partial reversal of the nitrogen incorporation in a lanthanide nitride by cycling vacuum and nitrogen conditions in the sample chamber.