The Mendocino War was a conflict between the mainly Yuki tribes and white settlers in Mendocino County, California between July 1859 and January 18, 1860. It was caused by settler intrusion and slave raids on native lands and subsequent native retaliation, resulting in the deaths of hundreds of Yuki.

In 1859, a band of locally sponsored rangers led by Walter S. Jarboe, called the Eel River Rangers, raided the countryside in an effort to remove the natives from settler territory and move them onto the Nome Cult Farm, an area near the Mendocino Indian Reservation. By the time the Eel River Rangers were disbanded in 1860, Jarboe and his men had killed 283 warriors, captured 292, killed countless women and children, and only suffered 5 casualties themselves in just 23 engagements. The bill to the state for the rangers' services amounted to $11,143.43. Scholars, however, state that the damage to the area and natives in particular was even higher than reported, especially given the vast number of raiding parties formed outside of the Eel River Rangers.

Other settlers formed their own raiding parties against the natives, joining Jarboe in his mission to rid Round Valley of its native population. Those that survived were moved to the Nome Cult Farm, where they experienced hardships typical of the reservation system of the day. After the conflict, contemporaries claimed that the conflict was more of a slaughter than a war, and later historians have labeled it a genocide.

Round Valley, located in northeastern Mendocino County in Northern California, was home to various Native American tribes. The most populous of these local tribes were the Yuki, whose territory was roughly 1,100 square miles. The Yuki were not one political people; rather, they were several autonomous groups that shared both language and culture, with each community having its own leadership.

In 1853, California started its Indian Reservation System, which was headed by Thomas J. Henley (Superintendent of Indian Affairs), and by 1854 Round Valley was discovered by white settlers. Pierce Asbill, the first white man to see the territory, estimated that there were about 20,000 natives in the area at the time. Scholars now believe this number is a little high, but by 1856, there were 12,000 Native Americans in Round Valley. Although a few families moved into native territory, many of the settlers were rather likes of hunters, fugitives and drifters. In general, they were people who lived off of the land, who traveled to the area for its resources. In the same year, Thomas Henley sent Simmon Pena Storms to start the Nome Cult Farm. Originally meant to be a resting point for natives and people traveling to the Mendocino Indian Reservation, the Nome Cult Farm grew to become a reservation of its own, occupying 5,000 acres of northern Round Valley. This division of the 20,000 acre territory left over 15,000 acres for white settlement.

Despite the amount of land set aside for white settlement, the government had trouble stopping newcomers from settling all over the valley, including on the Nome Cult Farm and Mendocino Reservation. As settlers moved into what was designated native territory, it became hard for the natives to survive. Those that lived on the Nome Cult Farm lived a life of hardship. In a type of indentured servitude, the natives raised their crops but reaped little of the actual benefits. Natives were not protected but were subject to brutal treatment that included assaults, rape, murder, theft of their property, disease, and starvation. Many white settlers who encroached on native territory engaged in kidnapping, stealing Native women and children and subjugating them to servitude or sexual abuse. Natives at the Nome Cult Farm were overworked, and could even be killed if their work was not up to the standards of the reservation. White settlers continued to exploit native land, with many families fencing in thousands of acres each. They removed fences from the Nome Cult Farm and allowed their herds to graze on and through native land, some of which was already filled with crops. The California Reservation System, which was subject to corruption, fraud and misuse of federal funds, provided little recourse. As more settlers encroached on native land and resources, native food sources dried up on and around the reservations.

Since ranching methods at the time were not very advanced (barbed wire had not been invented), the settlers had trouble keeping their livestock on their land. Many tried to train their animals to stay in a certain area, but this was not always effective. Livestock often wandered, and the local terrain made matters worse. The territory was new, unfamiliar, and full of hazardous cliffs and predators, and many cattle and horses wandered off and died of natural causes. However, the settlers blamed the natives for any animal that went missing, believing that they were the targets of "Indian Depredations", holding public meetings to stir up animosity towards the natives. In retaliation, they continued their assaults on native land and resources. With no police force at hand, the reservation was powerless to stop local theft of native property or abductions of native people. Locals like Dryden Lacock even stated that settlers, including himself, were engaging in small raiding parties that killed "50-60 Indians a trip". Finally, on the brink of starvation and left with almost no options, the natives began to retaliate. In 1857, a Yuki shot a man named William Mantle while trying to cross the Eel River, and a white man named John McDaniel was killed in 1858. Both had been famous for crimes committed against Native Americans, and reports from the U.S. Army claim that the natives were provoked in both instances.

As tensions rose and natives began retaliating for crimes committed against them, the settlers petitioned the U.S. Army for aid. In 1859, the 6th Infantry Regiment led by Major Edward Johnson was called to Round Valley. Major Johnson sent Lieutenant Edward Dillon ahead with 17 men to scout the area and assess the situation. Lieutenant Dillon reported back that the settlers misrepresented the situation. Instead of settlers falling prey to natives, the settlers had in fact already killed hundreds of natives, whose hostile actions had been taken out of revenge or in an effort to survive. The problem, he reported, went all the way up the chain to Supt. Henley, who had been involved in organizing many of these raiding parties. In fact, Supt. Henley was in league with Judge Serranus C. Hastings (a former Iowa Supreme Court Justice), who helped him design plans for the removal of natives from the local territory. As part of their plan, they launched raiding parties and held town-hall style public gatherings where settlers aired their grievances, leading to increased racial prejudice and hatred towards the natives. Judge Hastings was also involved in real estate and livestock trade, and in one instance, the natives stole Judge Hastings's $2,000 stallion in retaliation for the beatings they received at the hands of Judge Hastings's ranch manager, H.L. Hall. Hall had been involved in many brutal assaults on natives. He complained to Lieutenant Dillon that the natives were stealing white supplies. Dillon urged Hall to let him handle the situation, but Hall ignored the command and took his own men raiding. By March 23, 1859, Hall and his men had killed about 240 natives. Dillon reported that Hall did not distinguish between guilty natives or innocent ones, and that his murders of even women and children were unprovoked. In fact, later on when Hall asked for soldiers at his property to protect his livestock, the soldiers refused to do anything to help him, since they were only ordered to defend a native onslaught, and they did not believe what was happening resembled a native attack. The natives faced a choice of either starving to death on the reservations that provided them with no food, or venturing off into the mountainous regions of Mendocino County and risk slaughter by local settlers.