Majority to get in? The original 13 states all unanimously bound themselves together into the very first compact under the Articles of Confederation. Arguably, the "American Union" was established earlier than that while still colonies under the Articles of Association 1774, the first act of the Continental Congress. All of the states approved the Articles of Confederation. Needed 9 of 13 states (2/3 majority) to ratify the change in form of government from the Articles to the Constitution, and all eventually ratified. Again, unanimous.

The first governing laws of the newly created United States of America were the Articles of Confederation and PERPETUAL Union. The intent was clear that the nation was to remain indivisible, permanently one country. The Constitution did not change the nature of the nation, only the form and operation of its federal government. Simply put, there were no terms for "leaving" because there was never to be any leaving. The Founders assumed that the states would check and nullify the federal government if and when it would be necessary, but never to just flat out say "we're leaving!" New England states threatened it early on during the war of 1812 and southern, Virginia led states laughed at such a notion (even Madison and Jefferson decried the claim of "secession"). How ironic that the southern slave states would make the same argument a few decades later after they lost power.