This article was updated to clarify the concept of the Posse Comitatus Act.

On Wednesday, President Donald Trump signed a memo to send National Guard troops to the U.S.-Mexico border. “The situation at the border has now reached a point of crisis,” Trump wrote in the memo. “The lawlessness that continues at our southern border is fundamentally incompatible with the safety, security, and sovereignty of the American people. My Administration has no choice but to act.”

There are a lot of questions right now about what these troops are supposed to be doing at the border, and what they are even allowed to do. While many of the specific details of the deployment remain unknown, we’ll do our best to answer all of your questions here.

What’s happening and where is it happening?

President Donald Trump is sending U.S. National Guard troops to the southern U.S. border. On Thursday Trump said he wanted to deploy between 2,000 and 4,000 troops on the border.

Can he do that?

Sort of. He’s the commander-in-chief of the U.S. military, but border-state governments will essentially be in charge of the deployments within their states—including whether they’ll agree to deploy troops in the first place—so Trump will have to cede much control here. Texas governor Greg Abbott is already on board, so we’ll definitely be seeing troops in Texas (California, however, is another story).

When will the troops get there?

Really soon. “We do hope the deployment begins immediately,” U.S. Secretary of Homeland Security Kirstjen Nielsen told reporters Wednesday. “We’re working with all haste.”

This sounds like a big deal. Is it a big deal?

Kind of. It’s a big deal anytime a U.S. president activates military troops to be deployed inside our nation’s own borders, but the National Guard won’t be able to do a whole lot along the border (which we’ll explain below), and this sort of thing has happened before. In 2014, then-Texas governor Rick Perry deployed about 1,000 National Guard troops to the border in an attempt to stymie what was at the time an overwhelming stream of Central American immigrants crossing into the United States (Perry framed it as a rebuttal to President Barack Obama’s perceived soft immigration enforcement efforts, and at the time he was considered a leading candidate to challenge for the presidency in 2016). Obama had also sent troops to the border, deploying about 1,200 troops in 2010 before scaling the deployment back in 2012. President George W. Bush rolled out the largest deployment to date, sending a whopping 6,000 troops to the border in 2006. Abbott extended Perry’s deployment order in 2015, and according to the Austin American-Statesman, about 100 Texas National Guardsmen currently remain at the border here. So, this is not an unusual move. But prominent Democrats and immigration activists are still concerned, calling the deployment of troops to the border unnecessary and decrying the further militarization of the border region.

Trump says he’s doing this because there’s an immigration crisis. Is there really a crisis?

It doesn’t seem like it. Well, at least not compared to past “border crises.” According to Customs and Border Patrol data, the number of apprehensions in 2017 reached its lowest point since 1971, and there were more than one million fewer people apprehended by Border Patrol last year (310,531) than there were in 2000, when 1,676,438 apprehensions were reported. But there was a slight uptick in apprehensions over the past two months. According to Border Patrol statistics, there were 26,666 apprehensions along the U.S.-Mexico border in February, compared to 37,393 in March.

What can National Guard troops even do here, anyway?

Not much!

Wait, why?

Posse Comitatus.

Um, say that again? 

Posse Comitatus!

…?

It’s a law that was enacted back in 1878, and it significantly limits the involvement of the U.S. military in civilian affairs. While the law does not govern a deployment by governors, the deployments under Obama and Bush seemed to be mindful of the concept. The act states: “Whoever, except in cases and under circumstances expressly authorized by the Constitution or Act of Congress, willfully uses any part of the Army or the Air Force as a posse comitatus or otherwise to execute the laws shall be fined under this title or imprisoned not more than two years, or both.” In the early days of the United States, the federal government frequently called upon the military to quell large strikes, disturbances, or insurrections, like the Whiskey Riot, according to a 2012 report by the Congressional Research Service. That changed in the years following the Civil War. By 1878, most political restrictions that had been brought against former confederate states had been lifted, but federal troops were still being used in a law enforcement capacity in those states, prompting Congress to pass the Posse Comitatus Act.

Oh. So, uh, what’d the troops do those other times they were deployed to the border?

During the deployment under Bush, troops were focused on training, conducting surveillance, and building fences and barriers. Under Obama’s deployment, troops deployed a fleet of UH-72 Lakota helicopters equipped with infrared cameras to see better at night and to identify immigrants traveling beneath the cover of heavy brush below. Shortly after Perry deployed troops to the border, the Washington Post reported that troops “saw little activity after taking their positions in portable towers and Border Patrol vehicles along the dirt roads and levees that overlooked the dense brush near the Rio Grande,” and “found themselves fighting boredom.” Some listened to music to kill time while staring at the brush along the riverbank.

Don’t these guys have anything better to do than stare at the border brush and listen to tunes?

It depends on whom you ask. Border security has arguably been the highest priority for Trump and some members of his administration, so to them it would seem to be a proper allocation of resources to send troops to the border. Others aren’t so sure. According to the New York Times, military leaders have never really liked the idea of sending troops to the border. “There is a significant opportunity cost,” James G. Stavridis, a retired four-star admiral who commanded United States forces in Europe and Latin America, told the Times, adding that the troops deployed to the border would “miss important training opportunities for their real primary mission—combat.” In 2011, the Government Office of Accountability published a report expressing concern from within the Department of Defense about “the absence of a comprehensive strategy” for the troops deployed to the border. The report also raised concerns that deploying troops to the border would hurt the U.S.’s relationship with Mexico.

What about Mexico? How do they feel about the U.S. putting armed troops on the border?

Mexico doesn’t like it. “It’s certainly not something that the Mexican government welcomes,” Ambassador Gerónimo Gutiérrez told CNN on Wednesday. That’s not surprising given the heightened tensions lately between Mexico and the United States. Along with the ongoing “who’s going to pay for the wall?” dispute and Trump’s unsubtle diplomatic strategy of openly hammering Mexico on Twitter for allowing immigrants to travel from and through it on their way to the United States, this could potentially become yet another flash point in an already rocky relationship with an important ally.

I’m a taxpaying citizen of the U-S-of-A. What’s this going to cost? 

Probably a lot. There are still a ton of details that need to be ironed out regarding this current deployment, but past deployments have cost millions of dollars. Perry’s 2014 deployment reportedly cost $12 million a month. The price tag for Obama’s 2010 activation of the National Guard was a cool $500 million. Bush’s massive border troops rollout, meanwhile, cost $1.2 billion. According to Bustle’s math, the average taxpayer cost per troop on the border has been roughly the same throughout each deployment: $100,000.

How many troops will be going to the border?

We don’t know yet. But on Thursday Trump said he anticipates deploying between 2,000 and 4,000 troops. Past deployments have ranged from 1,000 to 6,000 troops.

How long will they be there?

We don’t know. Troops have been there in one form or another for a good chunk of the last decade, but the timeline is unclear for this particular deployment.

I’m a U.S. citizen living in the border region. Should I be worried? What does this mean for me?

You’re going to be okay. Might even make some new friends. While you may have already been concerned with the increasing militarization around and surveillance of your neighborhood, at this point the arrival of National Guard troops is just more of the same for you guys.

I’m an undocumented person currently living in the border region, or considering an illegal crossing into the United States. Should I be worried? What does this mean for me?

NBC News reported on Wednesday that administration officials said troops won’t have physical contact with any immigrants and they won’t be tasked with processing them at the border. Because of Posse Comitatus, the troops are lawfully barred from detaining you or physically assisting in detainments. On the other hand, with the additional troops will almost certainly come an increase in surveillance along the border. This move also shows Trump’s continued commitment to “securing the border,” and he essentially equated illegal aliens with “gang members,” criminals, and drug traffickers in the memo, indicating that his view of undocumented people hasn’t changed a bit since he called Mexican immigrants “rapists” and criminals when he announced his presidential campaign back in 2015.

There’s some risk in activating armed troops within U.S. territory, right?

Of course. In 1997, U.S. Marines were conducting a drug surveillance mission along the border near Redford, Texas, when they shot and killed 18-year-old Ezequiel Hernandez Jr., an American student who was just trying to herd goats. And then there was the infamous Kent State massacre in 1970, when National Guard troops opened fire on unarmed protesters at Kent State University in Ohio, killing four. Trump’s erratic tendencies and harsh rhetoric has worried some that he might surpass previous deployments and send a massive number of troops to the border, increasing the risk involved. “All it takes is one mistake,” Dov S. Zakheim, the Pentagon’s top financial officer under Bush, told the New York Times. “Somebody fires. And then what?”