It is often believed that the common-law rule of birthright citizenship was that mere birth on the sovereign’s soil was sufficient to create such citizenship. That is incorrect. Although that statement is an approximation of the rule that usually gets the correct result, the precise common-law rule was birth on the sovereign’s soil to parents under the sovereign’s protection.
That is how Sir Edward Coke described the rule in Calvin’s Case, the leading common-law decision from 1608. Aliens from friendly countries with permission to be in the realm were under the sovereign’s temporary protection and owed in exchange a temporary allegiance to the sovereign. They were, while in the realm, natural subjects of the king. That is why their children born in the realm were natural-born subjects. In contrast, the children born of invading soldiers were not birthright subjects “although born upon [the king’s] soil,” because they were not born “under the ligeance of a subject” or “under the protection of the king.” That is, a natural-born subject is one born to another subject, a subject who was under the protection of the king. Invaders did not count, but aliens were subjects of the king if they were under his protection and, in exchange, explicitly or implicitly, swore allegiance to him.


