"As Territories became States that in turn joined the Union (ack 'Oklahoma') "
To be pedantic they started out with 13 States that then joined together to form the USA.
Mr Lincoln made it a club you can't leave.
Territories such as Kansas, OK, Alaska etc were administered by the US Govt until such times as they petitioned to become States - they were already in , they just took a measure of control and responsibility from the Feds