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President Donald Trump announced this week that he was sending the National Guard to the U.S.-Mexico border to fight illegal immigration, signing a proclamation Wednesday invoking a part of the U.S. Code called Title 32 to do so. He estimated on Thursday that he could send anywhere from 2,000 to 4,000 troops, and that they would remain indefinitely.

"Well probably keep them or a large portion of them until the wall is built," Trump told reporters aboard Air Force One.

Rep. Ted Lieu, D-Calif., argued earlier Thursday that Trump wasn't actually "sending" the National Guard to the border, but simply requesting individual states to do so.

Proclamation by @realDonaldTrump doesn't actually send National Guard troops to the border. Because @POTUS is using Title 32 authority--instead of federalizing the Guard under Title 10--this is a REQUEST for state Governors to send troops.



That means @JerryBrownGov can say no. https://t.co/zyNJjFixJj — Ted Lieu (@tedlieu) April 5, 2018

Does Trump have the authority to deploy the National Guard himself? Here's how it's worked before.

How can a president get troops to the border?

The military has no legal mandate to deal with immigration issues, but there are two ways for the federal government to use the National Guard to enhance the country's border efforts.

Under the provisions of U.S. Code Title 10, the one Lieu cited in his tweet, the movement of federally funded National Guard troops is controlled by the secretary of defense. This is in effect the "federalization" of the Guard.

However, under U.S. Code Title 32, which Trump used as the basis for his proclamation, the federally funded troops remain under state control, sent only at the determination of a participating state's governor.

Presidents Barack Obama and George W. Bush both used Title 32 to direct National Guard forces to the southern border. In addition, the authority of Title 10 has for decades been used to federally fund military groups fighting transnational gangs that attempt to cross U.S. borders, according to a 2012 Government Accountability Office report.

Why use Title 32, not Title 10?

Title 32 broadens the scope of what National Guard personnel can do at the border.

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National Guard troops under control of the federal government, as they would be under Title 10, are prohibited by the Posse Comitatus Act (PCA) from enforcing domestic laws. The PCA bars military forces from getting involved with domestic issues like immigration. The troops can only take a support role, such as training or loaning and operating equipment, as well as being involved in certain counterdrug and counterterrorism efforts.

State National Guard personnel are not subject to the PCA. In 2004, Congress passed a law that previous administrations have used to allow the federal government to fund National Guard troops participating in border security operations triggered by "homeland defense activity" under Title 32, according to a 2013 Congressional Research Report.

NBC News reported Wednesday that it is unlikely the Guard troops will have physical contact with immigrants at the border. The exact number of troops and how long they will be deployed to the border will be firmed up in the coming days, officials said.

Can states say no?

Border states can fund and send their own National Guard troops to secure their borders, though historically many have requested Title 32 efforts so that the federal government, not the states, picks up the tab.

They can also refuse to send troops, as then-Gov. Arnold Schwarzenegger, R-Calif., did in 2006 when Bush requested that his state more than double the number of National Guard personnel deployed to the border.

What are states saying about Trump's plan?

Homeland Security Secretary Kirstjen Nielsen said Wednesday the administration had already started talking with states about utilizing their National Guard.

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. — Doug Ducey (@dougducey) April 4, 2018

Arizona Gov. Doug Ducey and Texas Gov. Greg Abbott, both border-state Republicans, welcomed the deployments, while a spokesman for California Gov. Jerry Brown, a Democrat who has been sharply critical of the president, told reporters the request will be reviewed promptly.

Oregon’s Democratic Gov. Kate Brown said she would say no, as did fellow Democratic Montana Gov. Steve Bullock, The Associated Press reported.

Nevada Gov. Brian Sandoval, a Republican, indicated that he would oppose the request through a spokesperson Friday, according to The Associated Press. The spokesperson told the AP that Sandoval doesn't think it would be "an appropriate use" of the Nevada Guard.

If @realDonaldTrump asks me to deploy Oregon Guard troops to the Mexico border, I’ll say no. As Commander of Oregon’s Guard, I’m deeply troubled by Trump’s plan to militarize our border. — Governor Kate Brown (@OregonGovBrown) April 4, 2018

Why does the president want to send troops?

On Easter, the president started tweeting about dangerous "caravans" of migrants marching through Mexico toward the U.S. The group he referred to was actually a planned, annual procession of migrants fleeing violence in Central America organized by Pueblo Sin Fronteras. Some of the migrants planned to seek asylum in the U.S. as they were fleeing extreme violence in their home countries — and they would have been stopped or apprehended at the border accordingly, immigration experts said.

The president said this was a dangerous situation, and blamed congressional Democrats for not passing stricter immigration laws and funding his planned border wall. Later, Trump said he'd use the military to protect the border.

Meanwhile, the caravan has stalled nearly 1,000 miles from the U.S. border in the Mexican state of Oaxaca.

"The president is frustrated," said Nielsen on Thursday, speaking to reporters at the White House who asked whether this was in response to a Fox News segment that had aired before the president's tweets began. "He has been very clear that he wants to secure our border. He's been very clear that he wants to do that in a bipartisan way with Congress. I think what you're seeing is the president taking his job very seriously, in terms of securing our border and doing everything we can, without Congress, to do just that."

A day later, the Department of Homeland Security released a statement insisting that the situation on the southern border was dire.

“The crisis at our Southwest border is real," a spokesman said. "We saw a 203 percent increase from March 2017 compared to March 2018 and a 37 percent increase from last month to this month — the largest increase from month to month since 2011."

But attempted border crossings, overall, have hit record lows, a fact that Trump's administration has trumpeted as a win for his tough stance on immigration. "In FY17, CBP recorded the lowest level of illegal cross-border migration on record, as measured by apprehensions along the border and inadmissible encounters at U.S. ports of entry," according to a report by U.S. Customs and Border Protection.

Just over a month ago, the president boasted about border crossings hitting what he called a 45-year low.