President Donald Trump signed a proclamation on Wednesday ordering the deployment of the National Guard to the US-Mexico border to fight illegal immigration.
"The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people," Mr Trump wrote in a memo authorising the move, adding that his administration had "no choice but to act".
The announcement came hours after Mr Trump pledged "strong action today" on immigration and a day after he said he announced he wanted to use the military to secure the southern border until his long-promised, stalled border wall is erected.
Homeland Security Secretary Kirstjen Nielsen said she had been working with governors of the south-west border states to develop agreements on where and how many Guardsmen will be deployed.
She suggested some troops could begin arriving as soon as Wednesday night, though other administration officials cautioned that details on troop levels, locations and timing were still being worked out.
Mr Trump has been frustrated by slow action on building his "big, beautiful wall" along the Mexican border - the signature promise of his campaign - as well as a recent uptick in illegal border crossings, which had plunged during the early months of his presidency, giving Mr Trump an accomplishment to point to when he had few.
Federal law prohibits the use of active-duty service members for law enforcement inside the US, unless specifically authorised by Congress. But over the past 12 years, presidents have twice sent National Guard troops to the border to bolster security and assist with surveillance and other support.
Ms Nielsen said the effort would be similar to a 2006 operation in which President George W Bush deployed troops to help US Customs and Border Protection personnel with non-law enforcement duties while additional border agents were hired and trained. President Barack Obama also sent about 1,200 troops in 2010 to beef up efforts against drug smuggling and illegal immigration.
Ms Nielsen said her department had developed a list of locations where it would like assistance on things such as aerial surveillance and other support, and was discussing with the governors how to facilitate the plans. She declined to say how many personnel would be needed or how much the operation would cost, but she insisted, "It will be as many as is needed to fill the gaps that we have today."
One congressional aide said that politicians anticipate 300 to 1,200 troops will be deployed and that the cost was expected to be at least 60 million dollars to 120 million dollars (£42m-£85m) a year. The Pentagon would probably need authorisation from Congress for any funding beyond a few months, said the aide.
Governors of the four US states bordering Mexico were largely supportive of the move. The office of California Governor Jerry Brown, a Democrat who has sparred with Trump on immigration issues, said any federal request would be promptly reviewed to determine how the state could best offer its assistance.
But in Mexico, senators urged President Enrique Pena Nieto to temporarily suspend cooperation with the US on immigration and security issues. In a non-binding statement approved unanimously Wednesday, the senators asked Mexico's government to freeze joint efforts "in the fight against transnational organised crime" until Mr Trump starts acting "with the civility and respect that the people of Mexico deserve".
Mr Trump spent the first months of his presidency bragging about a dramatic drop in illegal border crossings, which some Department of Homeland Security (DHS) officials had even dubbed the "Trump effect". Indeed, arrests at the border last April were at the lowest level since DHS was created in 2003, and the 2017 fiscal year saw a 45-year low for Border Patrol arrests.
But the numbers have been slowly ticking up since last April and are now on par with many months of the Obama administration. New statistics released ON Wednesday show about 50,000 arrests of people trying to cross the southwest border last month, a 37% increase from the previous month, and a 203% increase compared to March 2017. The monthly increase follows typical seasonal fluctuations.