Because debate over the criminalization of border crossings marks new and unfamiliar terrain for most Democratic politicians, misinformation and hot takes abound. To separate fact from fiction, here’s a guide to everything you need to know about Castro’s proposal to decriminalize border-crossing violations.

What was once the province of legal groups and the progressive wing of the immigrant rights movement is now rapidly becoming a consensus position among Democrats hoping to challenge President Donald Trump ― who used the law to systematically split up migrant families at the border in a widely deplored seven-week experiment last year.

Should crossing the border without permission be a criminal offense? This had not been a major part of the political discussion until last week, when Democratic presidential hopeful Julián Castro thrust his proposal to decriminalize border crossings before a national audience during the Democratic presidential primary debates.

What would it mean to decriminalize unauthorized border crossings?

It basically means the United States would use one legal system instead of two for going after people who cross the border without permission.

The United States governs most immigration violations under civil law rather than criminal law. When the government accuses someone of living in the country without authorization, that migrant is not being accused of a crime and won’t be punished with a prison sentence. But under the civil enforcement system, unauthorized migrants face the penalty of deportation, and the government holds tens of thousands of migrants in jail-like civil detention facilities on a daily basis. Migrant detention centers, in theory, differ from jails because they are not intended to punish but are instead meant to guarantee that a pending deportation will take place. The vast majority of undocumented immigrants arrested, detained and deported from the United States are processed by the civil system.

But crossing the border without inspection is also a federal crime. The first offense, called illegal entry, is classed as a misdemeanor punishable by up to six months in prison. Subsequent offenses can be punished with sentences up to two years for people without serious criminal records and up to two decades for people facing enhancements to their sentences because of more serious offenses.

Decriminalizing unauthorized border crossings would basically mean that instead of using both of these systems, we would rely only on the civil system.

Isn’t this the same thing as open borders?

No. Congress already pumps more money into civil immigration enforcement than all other federal law enforcement agencies combined. This proposal would not affect the budgets or enforcement efforts of immigration agencies like the Border Patrol or Immigration and Customs and Enforcement.

How long has illegal entry been a federal crime?

Congress first adopted the law criminalizing illegal entry in 1929, passing a measure put forth by segregationist Sen. Coleman Blease of South Carolina. The law played a major role in the early expansion of the federal criminal system, according to historian Kelly Lytle Hernández, driving the construction of three federal penitentiaries along the U.S.-Mexico border in the 1930s.

Over the years, the law has remained on the books and Congress has periodically fortified it with enhancements to ratchet up prison sentences for repeat offenders.

Then why are people talking about this now all the sudden?

The short answer is that the Trump administration used this little-discussed law to implement its family separation policy at the border last year. To scare away the surge of Central American migrants traveling together as families, the Justice Department would prosecute parents for the crime of illegal entry, routing them into federal jails run by the U.S. Marshals Service, and leaving their now-unaccompanied children in the hands of separate immigration agencies ― first Border Patrol and then temporary shelters run by the Department of Health and Human Services’ Office of Refugee Resettlement.

The longer answer is that the criminal immigration system has mushroomed exponentially over the last 15 years ― despite the fact that unauthorized crossings plummeted over the sharpest period of expansion, largely for unrelated demographic reasons.

The Justice Department has consistently prosecuted illegal entry and the felony version of reentry over the years, but it was a small part of the enforcement system until the George W. Bush administration. Instead, during the peak of mass migration from Mexico in the 1990s, the Border Patrol routinely walked unauthorized migrants back to the border after arresting them. Under that system, agents sometimes caught and removed the same person multiple times in a single day. So around the time Congress created the Department of Homeland Security in 2002, top leaders within border enforcement began rethinking the punishments they would mete out to the migrants they apprehended under what Customs and Border Protection refers to as its “consequence delivery system.” One of the punishments identified was the power to refer migrants for prosecution.

At around the same time, a large number of Central American and other non-Mexican migrants began crossing through the border sector of Del Rio, Texas. That created a problem for Border Patrol because it takes time to deport nationals from countries other than Mexico or Canada. That meant the Border Patrol had to transfer the detainees to Immigration and Customs Enforcement, but ICE lacked the bed space to hold them. Consequently, immigration authorities had no choice but to release the migrants with notices to appear in immigration court.

To resolve the problem, the departments of Justice and Homeland Security teamed up to prosecute the non-Mexican migrants for the crime of illegal entry, allowing federal authorities to rout them into jails run by the U.S. Marshals Service before their transfers to ICE. Under the 2005 program called “Operation Streamline,” the Justice Department began prosecuting migrants en masse for the crime of illegal entry, collapsing the process ― from meeting with a public defender to initial hearing and sentencing ― into a window of a few hours. Despite the due process concerns raised by the system, DHS expanded Streamline in the following years.

By 2008, combined prosecutions for illegal entry and reentry had become the Justice Department’s priority, swallowing up about half the federal criminal docket. That remained true throughout Barack Obama’s presidency, leaving him with the unexpected legacy of locking up more people of color on federal criminal charges than any other president in modern history.

Sen. Amy Klobuchar and former U.S. Rep. Beto O’Rourke say we need this law to prosecute human smugglers and drug traffickers. Do we?

No. The law criminalizing illegal entry has nothing to do with anything other than illegal entry. We have other laws that criminalize offenses like human smuggling and drug trafficking.

It’s unclear why these two appear to be confused on this issue. O’Rourke lives very close to the federal courthouse in El Paso where these prosecutions take place. Klobuchar is a former prosecutor who should have a clear understanding of overlapping laws.

Who gets prosecuted for illegal entry then?

With the major exception of Trump’s family separation cases, the vast majority of people prosecuted for illegal entry are single adults. The federal government is not supposed to prosecute people under this system who are seeking asylum or other humanitarian relief, but HuffPost has witnessed this occur in multiple instances, and it was also documented in a report by DHS’ Office of Inspector General.

Border Patrol arrests are on track to top 1 million this year. Don’t we need this criminal immigration statute to enforce the law?

No. Families and unaccompanied children have been driving the dramatic rise in unauthorized crossings this year. The Justice Department is not supposed to prosecute either group under this law. The children have various legal protections that require their transfer to civil immigration authorities, and a federal judge has ordered the Trump administration to stop prosecuting parents traveling with their children for illegal entry in order to halt the family separations. The illegal entry statute applies to single adults, and unauthorized crossings of single adults remain at very low levels by historical standards.

Juliette Kayyemsays immigration prosecutions set “a baseline for a nation’s enforcement capabilities against unlawful entry.” She worked at the Department of Homeland Security and said she did her homework, so isn’t she right?

No. Despite the outsized toll that these prosecutions take on the resources of the federal criminal courts and incarceration system, the centerpiece of immigration enforcement is, was and continues to be the civil system.

This is very clear from the numbers. Last year, immigration prosecutions took up a whopping 57% of the federal criminal docket, with about 94,000 cases, according to the database kept by the Transactional Records Access Clearinghouse at Syracuse University. That figure, which includes more than 2,000 parents who were split up from their kids in the process, outpaces the previous peak under Obama in 2013 and may have set a record.

But in 2018, the Border Patrol arrested nearly 400,000 people and ICE removed about 95,000 people from the interior of the country. That means the Justice Department, for all its obsession with prosecuting migrants under the “zero tolerance” policy implemented by former Attorney General Jeff Sessions, only processed about one-fifth of the number of people through the criminal justice system that the Border Patrol and ICE pushed through the civil system.

In the two decades before the advent of Operation Streamline, the immigrant-prosecution system barely figured in the larger enforcement picture. In the 1990s, when arrests for unauthorized crossings routinely topped 1 million per year, immigration prosecutions for illegal entry and reentry combined never surpassed 12,000 cases in a single year. Throughout the late 1980s and all of the 1990s, the Justice Department’s caseload of prosecutions for border-crossing violations never reached one-hundredth of the cases handled by civil immigration authorities.