California in the 1850s operated under explicit instructions. In 1851, the state's civilian governor declared that "a war of extermination will continue to be waged, until the Indian race becomes extinct." This was not rhetoric; it was law. An 1851 legislative measure gave settlers the right to organize armed groups to kill Native Americans and submit their expenses to the government for reimbursement. By 1852, California had authorized over one million dollars in such claims. The Gold Rush had flooded the region with 300,000 newcomers in barely a decade, colliding with roughly the same number of Native Americans who had lived there for millennia. Round Valley's isolation—difficult terrain, no easy routes—had briefly protected the Yuki who called it home.

Asbill's party proceeded to a creek bed where they encountered a large Yuki settlement. The six Missourians had horses, superior weapons, and focused intent. The Yuki had none of these. In the violence that followed, approximately 40 Yuki people died. No shots had been fired at the explorers. No theft had occurred. No provocation existed beyond the presence of Native Americans on land the explorers wanted. When it ended, Asbill stayed. He hunted the valley. He learned its terrain. And by 1855, he had added another enterprise to his resume: kidnapping Yuki women and selling them to non-Indian men outside the valley. Records indicate he trafficked at least 35 women in this manner.

Round Valley's remoteness, which had once protected the Yuki, became their prison. As settlers pushed other Native groups off their lands throughout Northern California, refugees converged on the isolated basin. The native population swelled to 20,000 while white settlers numbered only a few dozen. This concentration made the Yuki vulnerable to what came next. The 1850 California law "Act for the Government and Protection of Indians" legalized the kidnapping and forced servitude of Native Americans by white settlers. Slave raids became routine. Violence escalated into what historians call the Round Valley Settler Massacres of 1856-1859 and the Mendocino War—coordinated attacks designed to drive survivors from the valley entirely.

By 1860, every remaining Yuki had been forced onto a reservation. The violence had achieved its purpose: the valley was open for ranching and farming. But the dispossession continued. In the 1880s, settlers began appropriating reservation lands, triggering what became known as the Round Valley War—another cycle of conflict that stripped away more territory and more lives. Each "war" followed the same pattern: settlers encroached, authorities looked away, and Indigenous people lost ground. The Yuki, who had numbered in the thousands when Asbill first spotted movement in the grass, were reduced to a remnant population clinging to a fraction of their ancestral lands.

Neither Asbill nor his companions faced charges. The killing of "nonthreatening Indians," as the historical record describes them, was not considered a crime in 1850s California. No monument marks the creek bed where 40 people died. Round Valley today is home to the Round Valley Indian Tribes, a confederation that includes Yuki descendants alongside members of other displaced nations. The landscape looks peaceful—oak-studded hills, cattle pastures, the Eel River winding through. From the air, nothing indicates what happened here. The violence was so thorough, and so officially sanctioned, that it left no physical trace. Only the written record remains, preserved in the dry language of government reports and the academic studies that historians like Benjamin Madley have compiled under the title An American Genocide.