Arizona and Texas announced yesterday they were preparing to deploy National Guard members to the U.S.-Mexico border in response to President Trump’s call for more border security. Arizona Gov. Doug Ducey said about 150 Guard members would deploy next week, while Texas National Guard officials said Friday evening there will be 250 Texas National Guard troops deployed within the next 72 hours. 

Defense Secretary James Mattis Friday night approved paying for up to 4,000 National Guard personnel from the Pentagon budget through the end of September. A Defense Department memo says the National Guard personnel will not perform law enforcement functions or “interact with migrants or other persons detained” without Mattis’ approval. It said, “arming will be limited to circumstances that might require self-defense,” but it did not further define that.

Mr. Trump told reporters Thursday he wants to send between 2,000 and 4,000 National Guard members to the border to help fight illegal immigration and drug trafficking. That would be lower than the roughly 6,000 National Guard members former President George W. Bush sent in 2006 during another border-security operation, though more than the 1,200 Guard members President Obama sent in 2010.

Texas National Guard Brig. Gen. Tracy R. Norris said it is “premature” to speculate on the cost of the additional troops. Texas already has 100 National Guard members stationed on the border under a state mission.

Department of Homeland Security officials have said Guard members could support Border Patrol agents and other law enforcement agencies. DHS Secretary Kirstjen Nielsen said this week Guard members could “help look at the technology, the surveillance,” and the department might ask for fleet mechanics. 

From 2006 to 2008, the Guard fixed vehicles, maintained roads, repaired fences and performed ground surveillance. Its second mission in 2010 and 2011 involved more aerial surveillance and intelligence work.

Former Texas Gov. Rick Perry, now Mr. Trump’s energy secretary, also sent about 1,000 Guard members to the border in 2014 in response to a surge in the number of unaccompanied immigrant children crossing the Rio Grande, the river that separates the U.S. and Mexico in the state.