In January 1847, the area became controlled by the United States following the [[conquest of California]] as part of the [[Mexican–American War|Mexican-American War]].{{Citation needed|date=January 2024}} This was formalised by the [[Treaty of Cahuenga]] that month. Under the [[Treaty of Guadalupe Hidalgo]] in 1848, the United States recognised the existing land tenure, and took formal control of the land. It ruled it under a [[Interim government of California|military administration]] until a new civilian body was established in December 1849, which became the state of [[California]] in September 1850. In February 1850, the interim California government established [[Los Angeles County, California|Los Angeles County]]. (The earlier Los Angeles municipal government did not cover today's Ontario.)<ref>{{Cite journal |last1=Guinn |first1=J. M. |last2=Stearns |first2=Abel |last3=Valdez |first3=Bacillo |last4=Herrera |first4=Jose M. |date=1907 |title=FROM PUEBLO TO CIUDAD. The Municipal and Territorial Expansion of Los Angeles |url=https://www.jstor.org/stable/41168646 |journal=Annual Publication of the Historical Society of Southern California |volume=7 |issue=2/3 |pages=216–221 |doi=10.2307/41168646 |issn=2162-9145 |jstor=41168646}}</ref>
The new Californian administration soon began a war of extermination against the Tongva, which came to be known as being part of the [[California genocide]].<ref name=":0" /> 1850's [[Act for the Government and Protection of Indians]] ensured that slavery of the people it covered remained legal.<ref>[https://books.google.com/books?id=yLbNA_3CdcQC&dq=%22Act+for+the+Government+and+Protection+of+Indians%22&pg=PA822 '''Compiled laws of the State of California: containing all the acts of the Legislature of a public and general nature, now in force, passed at the sessions of 1850-51-52-53''', Benicia, S. Garfeilde, 1853. pp. 822-825 An Act for the Government and Protection of Indians]</ref>
