Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
reinforcement [riːɪnˈfɔːsmənt] n (strengthening) укрепле́ние; (action) усиле́ние
reinforcements
npl (MIL) подкрепле́ние ntsg
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.