President Trump did not actually order the National Guard to the U.S.-Mexico border in the proclamation he signed Wednesday evening; he ordered Defense Secretary James Mattis to "request use of National Guard personnel to assist in fulfilling this mission." That's because governors command their state's National Guard — as the governors alluded to in their reactions to Trump's memo.
New Mexico Gov. Susana Martinez (R) "appreciates the administration's efforts to bring states to the table as they go about taking steps to better secure our border," said her spokesman, Michael Lonergan. Texas Gov. Greg Abbott (R) noted that in his time as governor, and as far back as 2014, "Texas has maintained a continuous presence of National Guard members along the border," at a cost of millions of dollars. California National Guard spokesman Lt. Col. Tom Keegan said on behalf of Gov. Jerry Brown (D) that California will "promptly" review Trump's request, and "we look forward to more detail, including funding, duration, and end state."
Trump could theoretically take the rare step of federalizing the National Guard, though it "requires specific congressional authorization before the president can use the military in a domestic law enforcement capacity," University of Texas law professor Steve Vladeck tells Vox. But assuming he sticks with asking states to deploy the Guard, bypassing the Posse Comitatus Act, the states foot the bill — unless they come to some agreement with the feds. "In 2006, [California] Gov. Arnold Schwarzenegger sent 1,000 additional National Guard troops to the border in response to a request by President George W. Bush, but only after a protracted dispute over who would pay for it," the San Francisco Chronicle reports.