“I have always been taught that Texas, because it was a free republic when it willingly joined the union, could leave whenever it decided to do so”
Tell that to the Southern States that WILLINGLY JOINED the Union. When they tried to leave Lincoln sent his murdering hoardes South and 4 years later almost a million Americans were dead. All the Founding Fathers believed in secession. Every last one of them. The murdering madman Lincoln said to hell with Jefferson, Madison and Washington. They were all wrong. He was right. We have all been slaves to the Federal government since April 1865, when Lee surrendered to Grant.
that's different...Texas was an independent republic, actually a separate country when it elected to join the union. If Canada elected to join the union, I do believe that they would have the right to unjoin it down the road.