President Donald Trump signed a proclamation on Wednesday directing agencies deploy the National Guard to the southwest border.
"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.
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.
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.
Trump said in a statement on Wednesday that the, "Situation at the border has now reached a point of crisis."
"The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people," Trump continued in the statement. "My Administration has no choice but to act."