Little known fact
In the Articles of Confederation and Perpetual Union, the first Constitution of the United States, in effect from 1781 to 1789, Canada was invited to join the Union of the newly independent United States. Article XI read ...
Canada acceding to this confederation, and adjoining in the measures of the united States, shall be admitted into, and entitled to all the advantages of this union; but no other colony shall be admitted into the same, unless such admission be agreed to by nine [of thirteen] States.
Apparently, the Canadians wanted nothing to do with the upstart neighbors to the south.