According to the Articles of Confederation, Canada can join us if it wants.
And The Articles of Confederation are part of the organic law of the united States of America.
This has been re-affirmed by Congress as late as about 2002, I believe.
The “Organic laws” are the highest laws. In order, they are:
1) The Declaration of Independence
2) The Articles of Confederation
3) The Constitution
The Constitution?
How quaint! < /sarc>
Canada wouldn’t be so bad as long as we didn’t have to take Quebec.