Federal law says the president of the United States may only call state National Guard members "into Federal service" when certain specific conditions are met, such as when "there is a rebellion or danger of rebellion against" the federal government, or when "the President is unable with the regular forces to execute the laws of the United States."
According to President Donald Trump, he alone gets to decide when or if such conditions exist. Or, as Trump recently argued in a legal filing to the U.S. Supreme Court, "such decisions are committed to the discretion of the President and are unreviewable" by the federal courts.
