When Oregon became a state, some parts of the former Oregon Territory were added to Washington state.
The Oregon territory became part of U.S through the Oregon Treaty which was signed in 1846.
- The eastern part of the Oregon territory was partitioned off into the new territory of Idaho which gives Washington the shape it has today
In conclusion, what happened to Washington when Oregon became a state was that some parts of the former Oregon Territory were added to Washington state.
Read more about Oregon Treaty
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