Rubuta ci gaba don wannan sakin layi - A 1898, Rynning ya gano cewa yaƙin da Spain ya fara kuma tsohon abokinsa, Kanar Leonard Wood, yana tayar da rundunar sojan doki na sa kai, wanda daga baya aka san shi da Theodore Roosevelt's Rough Riders. Ya shiga aikin soja a matsayin mai zaman kansa kuma ya dawo daga yaki a matsayin mai kula da na biyu a cikin Bungiyar B, 1st United States Volunteer Cavalry Regiment. A lokacin yakin San Juan Heights
, Rynning ya dauki umurnin na American line bayan mutuwar Captain Buckey O'Neill. Bayan 'yan mintoci kaɗan sai ya jagoranci Rough Riders zuwa Kettle Hill kuma shi ne Ba'amurke na farko da ya isa taron. A karkashin mummunar wuta daga kusa da San Huan Hill, Rynning ya umarci mutanensa su yi amfani da tutar rundunar don tara wasu kuma a wannan lokacin ne tutar ta yi mummunan rauni. A yanzu haka Ma'aikatar Laburare da Tarihi ce ke adana tutar a ginin babban birnin jihar a Phoenix, Arizona. Bayan haka, Rynning ya shiga Theodore Roosevelt a cikin cajin San Juan Hill, wanda ya ƙare yakin da nasarar Amurka.