Roop County is a defunct county of Nevada. It was created as Lake County in 1861 as one of the original nine counties of Nevada. The name Lake County was chosen due to the many lakes in the area, including Honey Lake, Pyramid Lake, and Winnemucca Lake. 
In 1862 Lake County was renamed Roop County after Isaac Roop, governor of the unofficial "Provisional Territorial Government of Nevada Territory," which had previously existed in the same area.   When Roop County was created, the border between Nevada and California was poorly defined in Nevada's Organic Act, and much of Roop County, including Susanville, was claimed by California as part of Plumas County.  The establishment of Lake (later, Roop) County in northwestern Nevada created a boundary dispute with California. This has been called the Roop County War, the War of Injunctions, or the Sagebrush War. 
This resulted in the appointment of a joint California–Nevada boundary survey, with Surveyor-General J.F. Houghton acting for the State of California and Butler Ives, for the Territory of Nevada. The final surveyors report, accepted by California on April 4, 1864, and by Nevada on February 7, 1865, proved that the Honey Lake area was in California. The California portion of Roop County became part of the newly created Lassen County, California, in 1864. The remainder was ceded to Washoe County, Nevada. 
- Wren, Thomas (1904). A History of the State of Nevada: Its Resources and People. Lewis publishing Company. p. 102.
- Davis, William Newell, Jr. (September 1942). "The Territory of Nataqua: an Episode in Pioneer Government East of the Sierra". California Historical Society Quarterly. 1 (3): 225–28. JSTOR 25161008.
- "Creation of Nevada's Counties". Nevada Association of Counties. Archived from the original on February 22, 2012. Retrieved September 25, 2012.
- California v. Nevada, 44 U.S. 125 ( Supreme Court of the United States 1980) ("Nevada's Constitution stated that its boundary would proceed "in a North Westerly direction along [the oblique section of the] Eastern boundary line of the State of California to the forty third degree of Longitude West from Washington [and then] North along said forty third degree of West Longitude, and said Eastern boundary line of the State of California to the forty second degree of North Latitude. . . ." Nev.Const., Art. XIV, § 1 (1864). Although it turns out that the 43d degree of longitude west from Washington does not exactly coincide with the 120th meridian west of Greenwich—which was the north-south reference in the California Constitution—the Special Master concluded that the Congress that approved Nevada's Constitution was of the view that the two lines were identical. Certainly the language of the Nevada Constitution supports this conclusion by seeming to equate the 43d degree of longitude west of Washington with the eastern boundary of California.").
- Elliott, Russell R. (1987). History of Nevada. Omaha: U of Nebraska Press. p. 74. ISBN 0-8032-6715-0.
- General history and resources of Washoe County, Nevada, published under the auspices of the Nevada Educational Association. Sagebrush Press, 1969. 1888. p. 20.
- The Nevada Observer: History of Roop County
- Nevada Association of Counties: Creation of Nevada's Counties