Working as I do with a focus on the period between 1780 and 1830, the War of 1812 frequently drifts into focus. It is not a war with which I was very familiar when I started at this job, and I still think there’s a lot more I could know about it (although I have very little interest in all the movements of all the troops). Still, I am aware that one of the issues which led to the war was continued impressment by the British Navy of people who considered themselves to be citizens of the United States.
I do not know, and admittedly have not taken the time to discover, how exactly one became a citizen of any nation in the late 18th and early 19th centuries. Now, of course, we have citizenship applications and tests and ceremonies where you swear an oath, and afterwards you get a new passport. What was it like then?
Yesterday, while in the archives, I came across a letter which mentioned this question of what makes a person a citizen. It is written by a man living in Liverpool, England, to an acquaintance back in the U.S. The man in Liverpool worked for the U.S. Government, and must therefore have considered himself an American, despite having been born before the Revolution. It doesn’t answer my question completely, but it’s an excellent insight into the attitudes which led to a war between the U.S. and the United Kingdom.
“I observe what in America constitutes a citizen of the U. S. differs from what is here considered the Qualification; pro. Ex. a subject of this Country settled since the peace, in the Territories of the U.S. altho’ admitted there a Citizen is nonetheless still held here a subject of this Country.”
Source: James Maury to Thomas Jefferson,  November 1791. Papers of James Maury, 1769-1917, Accession #3888 and #3888-a, Albert and Shirley Small Special Collections Library, University of Virginia, Charlottesville, Va.