Washington (CNN) The way President Donald Trump describes it, he's sending in troops to meet hordes of immigrants amassing at the southern border.

In reality, the National Guard's tasks will likely include more construction than policing -- and many of the immigrants may be looking for border agents rather than avoiding them.

Trump will not be the first President to deploy National Guard troops to assist in border security -- in fact, both of his predecessors did so -- but it remains unclear exactly what they will be there to do.

Unlike the picture painted by Trump's dire descriptions of caravans of immigrants poised to storm the border, the number of people trying to cross the border illegally last fiscal year was roughly one-third of what it was in 2006, when President George W. Bush called in the National Guard. And there are close to two times as many Border Patrol agents than there were at that time.

Critics of the plan characterize it as a political stunt, an effort to give the appearance of strong border security to appease the President's frustration with the difficulties of getting his border wall and other aggressive immigration priorities through Congress.

In fact, a host of legal restrictions on what troops are actually allowed to do at the border paired with historically low levels of attempted illegal border crossings leave more questions than answers about what the troops at the border will actually be able to accomplish.

JUST WATCHED Trump may send up to 4,000 troops to border Replay More Videos ... MUST WATCH Trump may send up to 4,000 troops to border 02:59

And so far, the Department of Homeland Security and the administration have been slow to answer a slew of questions about how the mission will work -- including what the agents will do, whether they'll be armed, how much it will cost, who will pay for it, how many will deploy and how long they will stay -- saying the details are still being worked out with the border state governors that will oversee the project.

The administration has also avoided questions about how it views restrictions on the ability of the military to participate in law enforcement -- saying lawyers are still examining the issue.

"President Trump continues to use every cynical political trick in the book to ignite anti-immigrant fervor," House Democratic Leader Nancy Pelosi said in a statement. "His needless militarization of the border is motivated purely by political calculation, not our national security."

The 6,000 troops Bush deployed and 1,200 that President Barack Obama deployed cost roughly $1.3 billion combined.

Restrictions on the Guard's role

If Trump imagined armed troops greeting would-be border crossers, the reality may look more like troops building roads and watching video feeds.

A post-Civil War law explicitly prohibits the use of military troops for law enforcement, and a host of laws and Defense Department regulations have extended that law to further clarify that troops cannot be used to participate in activities such as making arrests and conducting searches, according to a Congressional Research Service analysis

Past presidents haven't tested these boundaries and therefore courts have not weighed in, though the Trump administration has thus far not answered explicitly whether it will keep troops from performing immigration enforcement duties. In a call with congressional staff after the deployment was announced Wednesday an administration staffer said the issue was being looked at. In a call with reporters, another official said lawyers were examining it.

"Lots of different options are available in terms of lawful and appropriate ways to utilize the National Guard and other state and federal resources," the official said.

Troops deployed by Bush and Obama conducted support for Border Patrol -- flying aerial missions, performing tasks that freed up Border Patrol agents to the border, and doing construction.

According to a report at the end of Bush's Operation Jump Start, National Guard troops freed up 581 Border Patrol agents, logged more than 28,600 flight hours and built 130 miles of road, fencing and barriers and repaired more than 1,100 miles of road in addition to helping with drug and immigrant interdiction.

The Department of Homeland Security has repeatedly said it is still working out the details of what it wants the National Guard to do with border state governors, who will have a heavy say in how the deployment actually works.

Border Patrol Assistant Chief Carry Huffman told CNN in an interview that some of the tasks the agency would like National Guard troops to do include flying aerial missions, monitoring surveillance feeds, vehicle maintenance and doing construction projects like building and maintaining access roads.

The goal is "helping us get more badges back to the border," Huffman said, "because there are jobs we have agents doing like watching surveillance cameras -- they're very helpful with doing those types of things."

The Defense Department also announced Thursday that the National Guards efforts for border security will include "aviation, engineer, surveillance, communications, vehicle maintenance and logistics support," Pentagon spokeswoman Dana White said in a statement.

A Homeland Security official said decisions are still being made.

"We are still evaluating and discussing with the border states the missions the Guardsmen will perform in support of federal law enforcement," the official said Thursday. "Decisions about equipment carried by Guardsman -- including firearms -- is dependent on their assigned missions and will be made in conjunction with the Department of Defense and the border governors."

Border security in context

Huffman said there were "concerning" trends in the number of border crossings, including an increasing share of families and children that have been arriving at the US border in recent years, even though overall border crossings are at historic low marks.

Though numbers have rebounded to be more consistent with the past few years, the number of crossings in fiscal year 2017 were at a nearly five-decade low.

Last fall, a Department of Homeland Security report concluded "the southwest land border is more difficult to illegally cross today than ever before."

When Bush deployed troops there were more than 1 million people apprehended at the southern border each year, and roughly 11,000 border agents, according to DHS data. In 2010, when Obama ordered a limited deployment, there were almost 450,000 apprehensions and close to 18,000 Border Patrol agents.

In fiscal year 2017, there were just over 300,000 apprehensions at the southern border and just under 19,500 Border Patrol agents.

JUST WATCHED Border sheriff: Guard deployment unnecessary Replay More Videos ... MUST WATCH Border sheriff: Guard deployment unnecessary 03:13

The caravan to which Trump was referring -- an annual event to draw attention to the plight of migrants -- was a group of roughly 1,000 migrants making their way through Mexico, many with the intent to reach border agents and ask for asylum.

In fact, many of the immigrants who try to cross the border either seek to get caught or arrive at a port of entry, because they can begin the process of claiming they have a right to protection in the US from persecution in their home country.

The rise in families and children Huffman cited has also occurred with a rise in the number of what Customs and Border Protection calls "inadmissables" -- people who arrive at a legal border crossing but do not have authorization to enter the US.

The administration complains that because of anti-trafficking laws and court rulings, they are required to spend resources caring for these vulnerable populations differently and must release them from detention centers quickly, after which they live in the US awaiting a court date that can be years in the future.

But it's unclear if the National Guard would have any role to play in that process.

"Until we're able to claim that we have established full operational control of the border, I think there's always a need to increase what we're doing," Huffman said of the deployment.