If you’re an unauthorized immigrant in the US, you’re committing a civil violation: being present in the US without a valid immigration status. That’s breaking a law, but it’s not a crime, in the same way that violating the speed limit isn’t a crime. If you’re arrested, you can be deported — a huge change to an immigrant’s life, but not technically a criminal punishment.
But if you cross the US/Mexico border between ports of entry without papers, you are committing a federal misdemeanor: illegal entry. And you can be jailed and fined in addition to getting deported.
In one respect, this system has been on the books since 1929, when illegal entry was first made a misdemeanor. But for most of the 20th century, it was kind of irrelevant. Most people who came into the US without papers weren’t tracked down and deported. Presidents generally decided that it wasn’t worth it to spend US attorneys’ time prosecuting endless misdemeanor illegal-entry cases. Those who were caught crossing the border were generally informally returned.
Under the Bush administration, however, as an independent immigration-enforcement system began to develop and mature, both civil immigration cases (in separate immigration courts) and widespread criminal illegal entry prosecutions became common. The result swamped federal criminal courts along the border. For the past several years, immigration offenses — illegal entry and reentry — have been the most common crimes for which people are convicted in US federal criminal courts. (In fiscal year 2016, immigration offenses made up a majority of all federal criminal prosecutions.) And the courts along the border where entrants are prosecuted are routinely the busiest in the country.
Key point on reversing it:
Proposing that illegal entry no longer be a federal crime is the policy equivalent of the “no human is illegal” slogan — a way to combat hawkish attitudes toward the “rule of law” by challenging the idea that migration ought to be a matter of crime and punishment to begin with. But it’s also a key justification for reversing the past few decades of border crackdown, by unpinning immigration enforcement — at least when it comes to unauthorized immigrants themselves — from crime.