Currently, the borders of Washington are roughly the same as they were when
Britain and the US had joint control of the Pacific Northwest.
Oregon Territory was created in 1853
Idaho Territory was created in 1863.
Olympia was chosen to be the capital of Washington.

one of the main reasons was definitely slavery. back in 1861, republican abraham lincoln was elected and ended up being a major influence in the slavery business. slavery greatly affected the texas economy because back in 1845 during the annexation of texas, slaves became a big part of the texas population. many texans that were in favor of the secession influenced groups through party ties.