When Wyoming Territory was formed in 1868, there were no major towns and the majority of citizens lived along the very new transcontinental railroad across the south. For simplicity, the state was divided into five large counties, Uinta, Carter (later Sweetwater), Carbon, Albany and Laramie, each running from the southern border to the northern border. The borders were arbitrary straight lines not based on geography. Because these counties were formed successively over a two-year period (1867-1869) by first the Dakota Territory and then Wyoming Territory, the east half of what is now Sublette County was actually part of Laramie, Carter and Sweetwater counties at different times. The west half was part of Idaho Territory until 1869 when it became part of Wyoming Territory and Uinta County.
As more people settled the area, the split of the Upper Green River Valley along the 110 west longitude became a real burden as neighbors had to do official business in two different far-off county seats, Evanston (later Kemmerer) and Green River (later Lander). There was also confusion over which county landowners had to pay taxes. Several ranchers who paid land taxes in Uinta County faced threats from Fremont County to sell the very same land for non-payment of taxes. This was ultimately resolved in 1907 by a survey of the border sponsored by both counties in which some land thought to be in Uinta was given to Fremont.
For reference, Pinedale, Cora, and Boulder are located east of 110 west longitude and Bondurant, Daniel, Big Piney and Marbleton are west. North to south, the dividing line roughly crosses Black Butte, Desmet Monument and 6 miles east of LaBarge.
The territory legislators always knew the original counties would be divided taking into account geography (mountain ranges and river drainages), but they first had to be settled by permanent residents and establish real and personal property that could be taxed.
By the time the territory was formed into the State of Wyoming in 1890, eight new counties had been formed, creating 13 total. This included the formation of Fremont County (1884) out of the northern majority of Sweetwater County. So the east half of the Upper Green River Valley was now in Fremont County with the county seat in Lander. From 1909 through 1913, eight new counties were formed. The last of these was Lincoln County formed by the legislature in 1911 and fully organized by 1913 with Kemmerer as the county seat. This was the first division of the original Uinta County placing the northern 80 percent of land in the new county including Cokeville, Afton, Jackson and the west half of the Upper Green River Valley. It was greatly contested by the citizens of Evanston as Uinta County was reduced to 20 percent of its original size. West-half residents of the Upper Green River Valley now conducted county business in Kemmerer instead of Evanston.