In 1859, the Washington Territory made a successful bid for statehood. While the main US transcontinental railway had passed to the north of the fledgling state, it had not done so by all that much, plus the lands of the Washington Territory had benefited from being along the southern spur of the wagon trail to Oregon for decades beforehand, with settlers not always wishing to go all the way to the Pacific. Lastly, expatriot Southerners of various stripes had also come across the border to settle. Washington was also along the main routes for driving Southern Cattle across the border into the USA.
In the process of transforming from territory to state, however, some adjustments were made to the borders. The unweildy jughandle of land in the northwest of the territory was lopped off and ceded to the Colorado Territory west of the 102nd meridian (with the 102nd meridian up to the Southern Fork of the
Republican River forming the western border of the state). Washington's northern border was modified slightly by starting with and following the Republican River, which runs very close to the 40th parallel, until it crosses south of said 40th parallel for the last time, then follows to 40th parallel to the border of Missouri, with Missouri's western border forming its eastern border. The southern border of the state of course was at 36-30, the international border with the Dominion of Southern America.
The new
State of Washington and surrounding nations, states, and territories.