The inspiration for Donald Trump's immigration policy, as Donald Trump himself has proudly told plenty of interviewers (and the moderators of last night's Fox Business presidential debate), is "Operation Wetback."

Trump didn't use the term in last night's debate. But that's what he was referring to:

Dwight Eisenhower, good president, great president, people liked him. I like Ike, right? The expression. I like Ike. Moved a million 1/2 illegal immigrants out of this country, moved them just beyond the border. They came back. Moved them again, beyond the border, they came back. Didn't like it. Moved them way south. They never came back. Dwight Eisenhower. You don't get nicer, you don't get friendlier. They moved a million 1/2 people out. We have no choice.

Trump's love for Operation Wetback makes sense — not just because the name, like a lot of other things Trump says, offends immigrants and Latinos in the year 2015. Operation Wetback, which took place during the Eisenhower administration, was the closest to mass deportation of unauthorized immigrants the US has ever actually come. Over the summer of 1954 and into 1955, hundreds of immigration agents swept through the southwestern United States, rounding up immigrants who were in the US without legal authorization and packing them into trucks, trains, planes, and ships to be sent back to Mexico. It was macho and militarized. It was very Trump-y.

Needless to say, Trump (like US officials in the mid-'50s) exaggerated the impact of Operation Wetback. And Trump might find the real story of the operation somewhat surprising: Behind the hyped, militarized Operation Wetback of 1954 were years of quieter (and arguably more effective) enforcement, as the US collaborated with the Mexican government to encourage the legal hiring of immigrant workers as a complement to a crackdown on illegal hiring. In fact, the greatest success of Operation Wetback is that it encouraged immigrants who'd been working in the US illegally to become legal — the sort of thing we might call "amnesty" today.

In the 1950s, unauthorized immigrants were seasonal workers, not residents

One crucial difference between today's immigration situation and the one six decades ago, is that most of the people Trump and other hard-liners want to deport are full-time residents of the United States. Their lives are here. That wasn't true of the immigrants who got deported to Mexico during Operation Wetback.

In 2014, a majority of unauthorized immigrants had been in the US for a decade or longer. Millions of unauthorized immigrants live in nuclear families with US-born children — and millions of others came here as children and have grown up in the United States.

This large population of unauthorized long-term residents is relatively new. Starting around 1996, the United States began seriously militarizing the border with Mexico to make it harder for people to cross without legal authorization. Previously, it had been easy for people to cross back and forth between families in Mexico and jobs in the United States. This "circular" unauthorized migration was especially useful to farmworkers, who would come to the US for the growing season, return to Mexico after it was over, and come back the next year.

Because it was also designed for agriculture, the main legal immigration program for Mexican laborers in the 1940s and '50s (the bracero program) worked the exact same way. Legal and unauthorized Mexican workers were going back and forth at the same time, to do the same jobs at different pay scales.

One reason this worked so easily was that one year's illegal worker could become the next year's legal one. Today, unauthorized immigrants who are caught and deported get barred from reentering the US legally for three or 10 years (or permanently). During the bracero era, no such penalty existed. An immigrant who'd worked as a farmworker without authorization one year was just as eligible as anyone else to get a bracero contract to work legally the next year — often more so, since he had agricultural experience (which was required for bracero workers).

Operation Wetback probably couldn't have happened without the cooperation of the Mexican government

US officials didn't announce Operation Wetback until the spring of 1954, as part of a big publicity push (more on that later). But the tactics they used developed slowly over the course of a decade.

As historians put it, the Immigration and Naturalization Service, the US agency responsible for immigration enforcement at the time, had a "parallel" set of policy goals. It wanted to encourage farmers to hire workers legally through the bracero program; conversely, it wanted to discourage farmers from hiring workers who'd crossed illegally. Mae Ngai, an immigration historian whose book Impossible Subjects is the best resource on immigration to the US before 1965, calls this "carrot and stick."

The INS started wielding the "stick" during the 1940s: ramping up enforcement along the southern border and trying to reduce illegal farm labor — largely at the behest of the Mexican government.

As Kelly Lytle Hernandez wrote in a 2006 article about the "binational history of Operation Wetback," Mexico needed workers to stay home and help develop Mexican land. The bracero program was designed to help control the number of Mexican workers leaving Mexico for the US every year: Workers were recruited locally and processed within Mexico, so they weren't depopulating the border regions during the growing season. But in 1943, when the US wasn't doing enough to discourage Mexicans from crossing illegally without bracero contracts, Mexico took a stand: "The Mexican Embassy warned the U. S. Department of State that if control was not established over the flow of illegal immigration into the U. S., Mexico would 'affect a complete revision of the [bracero] agreements.'" (Pretty much everyone involved in immigration policy at the time, American and Mexican alike, used "wetbacks" or "mojados" to refer to unauthorized immigrants.)

Mexican officials were eager to help the US with enforcement efforts. When the US started building a fence along some segments of the border in the late 1940s, Hernandez writes, "the governor of Baja California detailed Mexican soldiers to patrol and protect the fence 'during its erection.'"

One reason Donald Trump loves Operation Wetback so much is that, in his telling, the US government stopped deporting people just over the border (where they could easily return) and started sending them into the interior of Mexico instead. That definitely happened during the 1940s and '50s. And Trump gets the rationale absolutely correct: Immigrants dumped in the desert just over the border from the US would simply cross back over again, so Operation Wetback started sending Mexican immigrants who said they were from the interior of the country back where they came from (in practice, of course, this led some immigrants to lie about where they were from).

But the trains and trucks that carried immigrants back into the Mexican interior were handed off, at the border, from US officials to Mexican ones. It was Mexican officials who were in charge of taking people deep enough into the country that it would be harder for them to return.

This might come as a surprise to Donald Trump, who has said over and over that the Mexican government is deliberately sending people into the US, and has proposed punishing Mexico and its citizens with many different kinds of fees to fund a border wall. But it's in line with the history of US immigration policy, especially where Mexico is concerned. Mexico and the US have been partners in controlling migration far more often than they've been enemies. And US policy successes have often relied on Mexican help. It was Mexico, for example, that has aggressively apprehended Central American families trying to get to the US — ending last year's "border crisis."

Operation Wetback itself was largely a publicity stunt — with horrific humanitarian side effects

The US started getting much more aggressive with apprehensions, raids, and deportations during the early 1950s. But this might have had an impact officials didn't anticipate: Looking at the rising numbers of apprehensions of immigrants trying to enter and work illegally in the US, the American public started getting more alarmed about the "invasion" of illegal labor into the US. The US government became less interested in pleasing the Mexican government and more interested in reassuring its own citizens.

The result was what we now know as Operation Wetback.

Trump has it right in one important respect: He would have loved Operation Wetback. The commissioner of the INS at the time was General Joseph Swing, an old Army buddy of Eisenhower's. Swing's job was to militarize US immigration enforcement, and he did it well.

"I think he really did conceive of it as a military campaign," says Ngai.

Under Swing, federal agents started conducting massive raids complete with trucks and airplanes; one raid resulted in the apprehension of 1,000 immigrants over four days.

In April 1954, Attorney General Herbert Brownell announced the start of Operation Wetback. A task force of 800 agents started sweeping through southern California for immigrants working illegally. Throughout 1954 and '55, they swept through the Southwest. Agents conducted raids and set up roadblocks to apprehend immigrants.

Those they found were deported en masse: by train, by truck, by plane, and by cargo ship. The Mexican government still assisted the US on train runs — in fact, according to geographer Don Mitchell, the US delayed the start of Operation Wetback for 10 days so Mexico could get enough trains to the border. But Mexican soldiers weren't always as keen on keeping immigrants on the trains until the end of the route as US officials might like. Cargo ships provided a way for the US to deport immigrants deep into Mexico, while controlling the whole journey.

In Ngai's account, the government was a lot more concerned with showing that it was rounding up unauthorized immigrants than with the mechanics of how this actually went.

"I've seen this before with the INS; they just get this idea, 'Oh, we can do this!'" she says. "I think they tried to get naval vessels, but they couldn't. They ended up getting really crappy cargo ships."

Conditions for deported immigrants were horrifying. A later congressional investigation described conditions on one cargo ship as a "penal hell ship" and compared it to a slave ship on the Middle Passage. Immigrants who were dumped over the border in trucks didn't fare any better. They were shoved into trucks "like cows," driven 10 miles into Mexico, and unceremoniously dumped into the desert — often in punishing heat, without water. Families were torn apart.

The point of Operation Wetback was to conduct mass deportations quickly and on an impressive scale. It required the government to use all its resources on aggression. There wasn't any room to be humane.

Operation Wetback massively expanded legal immigration. That was the point.

Immigrants themselves weren't even the real target of Operation Wetback. The real target was the farmers who hired them — who had refused to use legal bracero labor until they were forced to.

"The INS was in this position where, for years, they had tried to carry out their responsibilities while also being sensitive to the farmers," says Ngai. "The kind-of-standard procedure for years was they'll do sweeps and raids, but not during harvest time. They used to go into farms [to conduct raids], but then the growers campaigned, so they had to go outside of farms."

The INS stopped being so accommodating during the Operation Wetback era — it's not a coincidence that the operation was announced in April and started in June, during the height of growing season. But at the same time, it expanded another practice: "drying out the wetbacks," or legalizing previously illegal workers so they could continue working for the same farmers.

On a couple of occasions in the late 1940s, the INS legalized thousands of unauthorized farmworkers on the spot at once. Another trick was called a "walk around the statute": INS officials would take workers just over the Mexican border, thus "returning" them to Mexico, then immediately allow them to reenter with bracero contracts as legal workers.

But in the 1950s, in the Operation Wetback era, "drying out the wetbacks" got institutionalized as "predesignated return." Farmers could designate any of their workers in advance to come back to the US legally on bracero contracts for the next growing season — whether or not they were legal this year.

The bracero program had started as a way for the US and Mexico to control the number of workers entering the US. "Predesignated return" made it something else: a program to create "a reservoir" (as Commissioner Swing said) of prescreened workers who could reenter the US year after year.

The very thing the Mexican government had worried about — the US recruiting hundreds of thousands of people away from the border — started happening. The number of workers on bracero contracts doubled between 1953 and 1955, largely because of Operation Wetback. And many of those legally contracted workers had previously worked illegally.

No one knows exactly how many people got deported — but it definitely wasn't 1.5 million

Even with more immigrants coming in legally, though, the net effect of Operation Wetback was almost certainly a decrease in immigrants in the US. In 1955, 400,000 or so workers came in on bracero contracts; annual US statistics, on the other hand, say the US apprehended 2.1 million unauthorized immigrants between 1953 and 1955.

That's even more immigrants than Donald Trump (and others who cite Operation Wetback as an immigration-enforcement success) say got deported over that time. They tend to go with 1 million or 1.5 million — the number of immigrants that the government announced got deported during Operation Wetback. The discrepancy shows just how hard it is to count deportations, and it shouldn't be surprising that the real number is probably a lot lower than 1.5 million, too.

Operation Wetback probably deported fewer immigrants than had been deported without hype the year before

During fiscal year 1954, the year Operation Wetback began, the US apprehended more than a million unauthorized immigrants. But Operation Wetback started toward the end of the fiscal year — and in fiscal year 1955, which started in October 1954, the government managed to apprehend fewer than 250,000 immigrants. (Enforcement continued to relax through the late 1950s, even though people were still coming to work illegally.)

On the other hand, in fiscal year 1953 — when the government was conducting large-scale raids but wasn't hyping them under the Operation Wetback banner — 875,000 immigrants were apprehended. Historians conclude that (in the words of Kelly Lytle Hernandez) "instead of being a major law enforcement campaign, the summer of 1954 can better be understood as a massive publicity campaign for what had happened the year before."

An untold number of immigrants got apprehended more than once. The government counted each time it caught an immigrant, which means immigrants caught twice, three times, or more inflated the government's numbers. (The same criticism gets made of immigration metrics today.) It's impossible to say how many different immigrants were apprehended from 1953 through 1955; Ngai, in her book, estimated the number was about 800,000.

The government counted each time it caught an immigrant, which means immigrants caught twice, three times, or more inflated the government's numbers. (The same criticism gets made of immigration metrics today.) It's impossible to say how many different immigrants were apprehended from 1953 through 1955; Ngai, in her book, estimated the number was about 800,000. The government claimed many immigrants returned "voluntarily" — and counted those as deportations too. Most of the official statements about Operation Wetback combine immigrants the government actually deported with immigrants who simply left when the government cracked down. Border Patrol historian Kitty Calavita found that about 60,000 immigrants left Texas voluntarily in the first 30 days of Operation Wetback — about as many as the government apprehended per month.

Today, "self-deportation" sounds silly — immigrants who've been living in the US for years and have no other home are extremely unlikely to pick up and leave of their own volition. But during Operation Wetback, leaving the US voluntarily meant you could go back over the border, get recruited, and come back as a legal worker without running the risk of getting deported to the interior. It's impossible to say how many of the 1.5 million Operation Wetback "deportees" were actually voluntary immigrants, and how many of them came back legally (or illegally) after their departure.

So for all of the hype of Operation Wetback, how many immigrants did it actually deport, never to return? It's unclear. The most cynical historians peg it at about 250,000. It's probably higher than that. But it could easily be 800,000 or fewer — about the same number of immigrants who got deported during the first two years of the Obama administration.

Operation Wetback definitely deported a bigger share of the nation's unauthorized population than got deported under Obama — there's no official count of how many unauthorized workers there were, but it wasn't anywhere near the 11 million we have today. But that's a reflection of how different the two periods are. Operation Wetback was designed to replace a group of illegal workers with legal (immigrant) ones, during a time when it was much easier to cross back and forth between the US and Mexico either legally or illegally.

The mass deportation of a Trump presidency would probably look a lot like Operation Wetback: hundreds of agents; big press conferences; trains, planes, trucks, and cargo ships. But it would be all stick, no carrot. And the stick wouldn't be for farmers — it would be for immigrants themselves, most of them with much deeper roots in the US than the braceros were ever allowed to have.