The court did not consider it the "key phrase".
1. The Second Amendment protects an individual right to possess a
firearm unconnected with service in a militia, and to use that arm for
traditionally lawful purposes, such as self-defense within the home.
Pp. 2–53.
(a) The Amendment’s prefatory clause announces a purpose, but
does not limit or expand the scope of the second part, the operative
clause. The operative clause’s text and history demonstrate that it
connotes an individual right to keep and bear arms