Kammala wannan jumla: Larsen ya shiga Rundunar Sojan Ruwa ta Amurka yana da shekaru 23; ya sami maki mafi kyau a aji na 75 a Makarantar 'Yan Takara. Ya umurci 3rd Battalion 5th Marines a lokacin Yaƙin Duniya na 1. Ba da daɗewa ba bayan da aka ba shi matsayi na babban kwamandan, ya jagoranci aikin haɗin gwiwa a ranar 25 ga Yuni, 1918, a lokacin yakin duniya na biyu.
Yaƙin Belleau Wood, wanda ya karɓi Citation Star. A ranar 4 ga Oktoba, 1918, an gano bangarorin bataliya yayin da suke aiki tare da rundunar sojojin Faransa ta hudu kusa da Rheims. 'Yan Jamus sun kai hari sau da yawa, amma duk da wannan rundunar ta yi nasarar riƙe duk ƙasar da suka kama. Ga jagorancin jarumi na Larsen a lokacin kare kwana biyu an ba shi lambar yabo ta Navy Cross da na biyu Citation Star. Ya sami Tauraruwa ta Uku a lokacin yakin; duk ukun daga baya aka canza su zuwa Taurari na Azurfa, wanda ya sa ya zama mai karɓar lambar yabo ta Azurfa uku. Ya halarci kowane babban aikin da Sojojin Ruwa suka yi a lokacin Yaƙin Duniya na Farko, kuma shi ne kawai memba na 3rd Battalion da ya yi aiki a lokacin yakin ba tare da rauni ba. A cikin 1919, Faransa ta ba shi kyautar Croix de guerre tare da dabino da Legion of Honor don ayyukansa a lokacin yaƙin.