When has the National Guard ever been used?

Quite frequently, especially in the last nine years. The National Guard is always present at any civil crisis in the US, and, ever since the Total Force Policy went into effect in 1976, have been subject to widescale deployment, the same as the regular forces. National Guard units made up a significant portion of the US force in Desert Storm. Force 2000 began rotating National Guard units into peacekeeping/presence missions, such as those in Bosnia and Kosovo. And the last few years of the war in Iraq have seen times when the number of National Guard soldiers in-country have surpassed the number of regulars.