(CNN) -- President Donald Trump will sign a proclamation directing agencies to deploy the National Guard to the southwest border, Homeland Secretary Kirstjen Nielsen announced Wednesday.
"The President has directed that the Department of Defense and the Department of Homeland Security work together with our governors to deploy the National Guard to our southwest border," Nielsen said at the White House.
The formal move follows days of public fuming by Trump about immigration policy, during which he has tweeted about immigration legislation in Congress, a caravan of migrants making its way through Mexico and what he calls weak border laws.
Nielsen, however, said that details of the plan were still being worked out and would not be coming on Wednesday.
"While plans are being finalized, it's our expectation that the National Guard will deploy personnel in support of CBP's border security mission," Nielsen said. "I will not provide the full details today."
The secretary added that she didn't want to "get ahead" of the state governors who will have a heavy say in the deployment of troops, but said it would be similar to past similar efforts.
"It will be strong, it will be as many as needed to fill the gaps today," she said.
Since the passage of the government spending package for the year -- which included $1.6 billion for border security but only a few dozen miles of new border barrier construction and a nearly equal amount of replacement fencing -- Trump has been critical of Congress for denying him more money.
Trump privately floated the idea of funding construction of a southern border wall through the US military budget in conversations with advisers, two sources confirmed to CNN last week -- a plan that faces likely insurmountable obstacles in Congress.
Texas Governor Greg Abbott released a statement that read in part:
"My top priority as Governor is ensuring the safety and security of Texans, and securing our southern border has always been essential to that mission," said Governor Abbott. "In my time as Governor, Texas has maintained a continuous presence of National Guard members along the border, and we’ve added hundreds of permanent Department of Public Safety troopers to the region. Today’s action by the Trump Administration reinforces Texas’ longstanding commitment to secure our southern border and uphold the Rule of Law, and I welcome the support. Going forward, Texas will continue to implement robust border security efforts, and this partnership will help ensure we are doing everything we can to stem the flow of illegal immigration. Today’s decision by the Trump Administration is consistent with previous actions taken to secure our southern border by President Barack Obama in 2010, and President George W. Bush in 2006."
Previous troop deployments
Sending National Guard troops to the border is not unprecedented. Both of Trump's predecessors also did so, though the moves were criticized for being costly and of limited effectiveness.
US law limits what the troops can actually do. Federal law prohibits the military from being used to enforce laws, meaning troops cannot actually participate in immigration enforcement. In the past, they've served support roles like training, construction and intelligence gathering.
From 2006-2008, President George W. Bush deployed 6,000 guardsmen to Southern border states, costing $1.2 billion and assisting with 11.7% of total apprehensions at the border and 9.4% of marijuana seized in that time.
From 2010-2012, President Barack Obama sent 1,200 guardsmen to the border to the tune of more than $110 million, and they assisted with 5.9% of the total apprehensions and 2.6% of the marijuana seizures on the border.