 Coulter akanyoresa muWestmoreland Guards, boka remasoja emunharaunda rakanga raunganidzwa muUnited States Army seKambani E yeChipiri Pennsylvania Volunteer Infantry Regiment. Raiva rimwe remauto maviri chete akatumirwa kubva kuPennsylvania kunoshumira kuMexico. Coulter akaona chiito pasi peGeneral Winfield Scott muKukombwa kweVera Cruz uye hondo dzakatevera dzeCerro Gordo, Contreras, Churubusco, uye Chapultepec, uye kubatwa nekugara kweMexico Citymuna 1847. Akashanda zvakananga pasi pomukuru mukuru weHondo yaVagari Vemo womunguva yemberi John W. Geary, murume waakanga asinganyanyi kuremekedza nemhaka yokuzvikudza kwake. 