The Constitution provides three bases for federalizing the Guard: "to execute the Laws of the Union, suppress Insurrections and repel Invasions." (Art. I, sec. 8, cl.15). It was the "execute the Laws of the Union" clause which President Eisenhower relied on when he federalized the state guard to enforce federal court integration orders.
Maybe, but I prefer the older system where the “State Militia/National Guard” were accountable to the State Governors, not the US President, directly. Federalism-REAL federalism should be the rule in our system, not the exception.