Beginning his career as an enlisted man, Hanneken served in the Banana Wars of the 1910s and 1920s. During the United States occupation of Haiti, he assassinated the resistance leader Charlemagne Péralte, for which he was awarded the Medal of Honor. Subsequently, granted a commission, Hanneken served in Haiti for several more months and was awarded a Navy Cross for killing another rebel leader. He received a second Navy Cross for his actions during the occupation of Nicaragua in the late 1920s.