(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 said troops could be sent to the border as soon as Wednesday night.
"It does mean that," Nielsen said when asked about how quickly this could be implemented. "It also means we will do it in conjunction with the governors."
But details of the plan were scarce -- Nielsen said details like numbers of those deployed, where they will go, exactly what they will do, how much it will cost and how long they will stay were still being worked out.
"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.
Arizona Gov. Doug Ducey tweeted, "Arizona welcomes the deployment of National Guard to the border. Washington has ignored this issue for too long and help is needed. For Arizona, it's all about public safety."
Asked if the move Wednesday came because of something the President saw on TV -- a reference to his ire about the migrant caravan after it was covered on Fox News -- Nielsen demurred.
"I think what's true is the President is frustrated. He's been very clear he wants to secure the border," Nielsen said. "I think what you're seeing is the President taking his job very seriously."
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.
Nielsen spent a large portion of her time at the White House podium criticizing Congress for failing to pass meaningful immigration laws or give DHS the authorities it has asked for, some of which would amount to dramatic changes to US law that could have implications in the courts and international law.
Nielsen also faced questions about why the announcement was timed Wednesday. Border numbers have been historically low, and a study from DHS this fall determined the border was at its most secure and most impervious to illegal immigration point in history.
The secretary cited a historic uptick in border crossings in spring months like April and advertising from smuggling groups, calling it in part "anticipating."
"Why not yesterday and tomorrow? Today is the day we want to start this process," Nielsen said. "The threat is real as I mentioned."
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.