George III issued a royal proclamation on October 7, 1763 that drew a line along the crest of the Appalachians and closed everything west of it to colonial settlement. It was written to settle what Britain had just won in the Seven Years' War. It set up four new governments, at Quebec, East Florida, West Florida and Grenada, and it told the governors that land reserved to Native Americans could only be bought by the Crown, at a public meeting, and never by private purchase or grant. The announcement was not popular.
There was a real crisis. The Treaty of Paris had been signed in February, handing Britain all of North America east of the Mississippi, and within months a loose confederation of nations around the Great Lakes attacked the forts the British had just taken over, in what is called Pontiac's war. London's problem was cost: garrisoning a continent was ruinous, another frontier war was the last thing the treasury wanted, and the line along the mountains was meant to keep settlers out of Native country, keep the fur trade running, and hold the army bill down.
What it did instead was anger the people who expected to be paid for the war in land. Virginia's governor had already issued warrants for hundreds of thousands of acres beyond the mountains, and land companies with London partners, including one George Washington had an interest in, were counting on the same ground. Squatters crossed anyway. The Crown had too few troops to turn them back. The Quebec Act of 1774 then extended Quebec's boundary over the Ohio country, overriding the line, and that act is named in the Declaration of Independence.
In Canada the document went the other way. Because it said Indigenous land could only be surrendered to the Crown, it became the foundation of the land claims Canadian courts still decide, and it is referred to in Section 25 of the Charter of Rights and Freedoms, which came into force in 1982. Treaties signed in the following decades, including the one negotiated at Niagara in 1764, followed the procedure the proclamation had set out.