How is the National Guard a militia?

Answer 1

Their status as a militia can be an issue of debate. However, as far as the technical aspects go, they are a militia in the sense that they are not a regular military formation, and that they can be called to serve the needs of non-federal entities. The Militia Act of 1903 defined the National Guard as the organisedmilitia, with other militia entities being the unorganisedmilitia.

A number of bills have been passed in the years since 1903 which have further separated the National Guard from the Constitutional definition of a militia, such as the National Defence Act of 1916 (which merged the National Guard into the US Army, and established the National Guard as the primary reserve force of the US Army, rather than the Army Reserve), and the Total Force Policy of 1974, which effectively defined the Regular Army, Army Reserve, and Army National Guard as one entity.

But, until the Supreme Court rules otherwise (which is highly unlikely), the Militia Act of 1903 stands.