Whakaotihia te rerenga e whai ake nei: I te tau 1846, i te timatanga o te Pakanga o Mexico-Amerika, i whakaingoatia a Roberts hei rangatira tuatahi, Mounted Rifles, i roto i te ope taua. I whakatairangahia ia hei rangatira i te tau 1847, a, i kite ia i te mahi i Veracruz, Cerro Gordo, Contreras,
Ko Churubusco, me te hopu i te taone nui o Mexico, ko Matamoros me te haerenga o Galajara. I Churubusco, i whakawhiwhia ia ki te rangatira mo te arahi i tetahi roopu whakamua o nga kaipupuri. I whakawhiwhia ia ki tetahi atu tohu, ki te rangatira rangatira i te tau 1847 mo te toa i te pakanga. Whai muri i te mutunga o nga pakanga, i mahi ia i te rohe me Washington, DC.