Col. Edwin Sumner conducted a campaign against the Navajo in 1851. He delegated Maj. Electus Backus to build Fort Defiance in the middle of Diné bikéyah . Col Sumner entered Canyon de Chelly but Navajo harassment attacks caused him to withdraw. In 1855 a treaty was signed at Laguna Negra by Manuelito and Zarcillos Largos for the Navajo and Henry L. Dodge , Governor Merriweather, and General John Garland. Two years later a severe drought hit the Navajo and decimated their livestock and agricultural resources, causing a famine in the winter of 1857-1858. A time of social disruption ensued. Surrounding tribes gradually increased their attacks on the weakened Navajo. The Navajo demanded in 1858 that Fort Defiance stop grazing their livestock on prime Navajo land. Soldiers shot 48 cattle and 8 horses belonging to Manuelito. Navajo warriors killed a servant of the commanding officer in retaliation for the killing of their people's livestock without compensation. The United States military demanded the murderer be brought to them, or they would begin a war. The Miles Campaign was sent against them, leaving Fort Defiance on September 9.
Question: How many years did the campaign against the Navajo last?
Answer: 7