How would Trump's promise of mass deportations of migrant work?

US President-elect Donald Trump has reaffirmed his plan to deport illegal immigrants on a large scale. He intends to declare a national emergency and use the military to carry out this action.

Trump believes the cost won't stop him. He said his main goal when he takes office in January is to make the border strong and powerful.

"It's not a question of a price tag. It's not - really, we have no choice," Trump told NBC News earlier this month.

But how would Trump's campaign pledge of mass deportations of migrants actually work and what are the hurdles he may face?

What are the legal challenges?

There are about 11 million undocumented immigrants in the US, according to the Department of Homeland Security and Pew Research. This number has remained the same since 2005.

Most have been here for over a decade. They have the right to due process, including a court hearing before they can be removed.

Increasing deportations would mean more cases in immigration courts. These courts are already facing long delays.

Most immigrants enter the deportation system through local law enforcement, not directly through Ice agents. However, many big cities and counties have laws that limit their help to Ice.

Trump wants to act against these "sanctuary cities". But the mix of local, state, and federal laws makes things complicated.

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Kathleen Bush-Joseph, a policy analyst at the Migration Policy Institute, or MPI, said co-operation with Ice is key. "It's much easier for Ice to pick someone up from a jail if local law enforcement co-operates, instead of having to go look for them," she said.

Ms Bush-Joseph mentioned an early August statement from the sheriff's offices of Florida's Broward and Palm Beach counties. They said they wouldn't help with any mass deportation plan.

"There are many others who would not co-operate with a Trump mass deportation plan," she said. "That makes it so much harder."

Any mass deportation plan will face legal challenges from activists. A 2022 Supreme Court ruling means courts can't stop immigration policies while challenges are being heard.

But can it be done, logistically?

If a US administration wanted to legally deport many people, they'd face huge challenges.

Under the Biden administration, deportations mainly target migrants caught at the border. Those deported from inside the US, away from the border, often have criminal records or are seen as threats to national security.

During Trump's time, raids on workplaces were stopped in 2021.

For over a decade, deportations from within the US have remained below 100,000. This is down from over 230,000 in the early Obama years.

"To raise that number to a million in a year would need a huge amount of resources," said Aaron Reichlin-Melnick, policy director at the American Immigration Council, to the BBC.

Experts doubt Ice's 20,000 agents and staff could find and track down such large numbers. This is according to the Trump campaign.

Mr Reichlin-Melnick said the deportation process is complex. It starts with finding and arresting undocumented migrants.

Detainees then need housing or alternative programs before seeing an immigration judge. The system is backed up for years.

Only then can detainees be removed from the US. This requires help from the country they're being deported to.

"Ice simply doesn't have the capacity to process millions," Mr Reichlin-Melnick said.

Trump suggested using the National Guard or military to help with deportations.

The US military's role in immigration has been limited to support at the US-Mexico border.

Trump hasn't given many details on how to carry out mass deportations.

In an interview with Time magazine, he mentioned building new migrant detention facilities. He also talked about giving police immunity from lawsuits.

He said there could be rewards for state and local police who help. Those who don't might miss out on benefits.

"We have to do this," he said. "It's not sustainable for our country."

Eric Ruark, director of research at NumbersUSA, said deportations from within the US need to be part of a bigger plan. This includes more border enforcement.

"You can't make progress inside if you're not stopping people at the border," he said. "That's what keeps them coming."

Mr Ruark also said cracking down on companies hiring undocumented migrants is key.

"They're coming for jobs," he said. "And they get them because we're not enforcing the law inside."

The financial and political costs

Experts think the cost of deporting a million people could be tens or hundreds of billions of dollars.

The Ice budget for 2023 was $420m for transportation and deportations. That year, they deported slightly more than 140,000 people.

Thousands of immigrants would be detained while waiting for court or deportation. Trump plans to build large camps for them.

More removal flights would be needed, possibly using military planes. Even small increases in these areas could cost a lot.

"Even a small change costs tens of millions, or hundreds of millions," Mr Reichlin-Melnick said. "A big change costs tens or hundreds of billions."

These costs would add to the expenses of other border enforcement plans. Trump wants to continue building the border wall, block fentanyl, and move troops to the border.

Adam Isacson, a migration and border expert, warns of the political cost of mass deportations. He works for the Washington Office on Latin America. He fears "nightmarish images" could harm a potential Trump administration's public image.

"Every community in the US would see people they know and love putting on buses," Mr Isacson said.

"You'd have some very painful images on TV of crying children, and families," he added. "All of that is incredibly bad press. It's family separation, but on steroids."

Have mass deportations happened before?

Under the four years of the previous Trump administration, around 1.5 million people were deported. This included both border and US internal deportations.

The Biden administration - which had deported about 1.1 million people up to February 2024 - is on track to match that, statistics show.

During the two terms of the Obama administration, more than three million people were deported. This led some to call Barack Obama the "deporter-in-chief".

The only historical comparison to a mass deportation program came in 1954. Then, as many as 1.3 million people were deported as part of Operation Wetback.

However, historians dispute this figure.

The program, under President Dwight Eisenhower, faced public opposition. This was due to the deportation of some US citizens and funding issues. It was largely discontinued by 1955.

Immigration experts say the earlier operation's focus on Mexican nationals and lack of due process makes it incomparable. "Those [deported in the 1950s] were single, Mexican men," said MPI's Kathleen Bush-Joseph.

"Now, the vast majority of people coming between ports of entry are from places that are not Mexico, or even northern Central America. It makes it so much harder to return them," she added.