Rubuta ci gaba ga wannan sakin layi - A 1846, a farkon yakin Mexican-American, an sake nada Roberts a matsayin mai ba da shawara na farko, Mounted Rifles, a cikin Rundunar Soja. An daukaka shi zuwa kyaftin a 1847, kuma ya ga aikin a Veracruz, Cerro Gordo, Contreras, Churubusco
, da kuma kama Mexico City, Matamoros da Galajara wuce. A Churubusco, an ba shi babban jagora don jagorancin ƙungiyar masu kai hare-hare. Ya sami ƙarin takardar shaidar, ga Lieutenant Colonel a 1847 don ƙarfin hali a lokacin yakin. Bayan karshen tashin hankali, ya yi aiki a kan iyaka da Washington, DC.