President-elect Donald Trump and his allies have promised to deport people without legal status en masse and to crack down on existing protections and programs that create pathways to legal status.This could impact roughly half a million undocumented immigrants in Illinois. They include some 30,000 recipients of DACA, or Deferred Action for Childhood Arrivals, also known as DREAM, and tens of thousands of recently arrived migrants.Chicago is widely known as a sanctuary city because it has laws in place that prohibit local officials from helping federal immigration agents enforce immigration law. Illinois has had a similar law in place since 2017 called the TRUST Act. Some suburban townships, including Evanston and Oak Park, also have their own versions.Advocates hope these sanctuary laws can help protect immigrants in coming years from deportation. But there are limits.What does it mean to be a sanctuary city?There is no legal definition, but it has become an unofficial term for any jurisdiction — whether a state, county or city — that discourages local law enforcement from cooperating with U.S. Immigration and Customs Enforcement, or ICE, officials. That usually means not collecting or providing information about residents’ immigration status — including anyone held at a local jail or prison — unless that person has a federal criminal warrant.How do Illinois’ and Chicago’s sanctuary laws protect undocumented immigrants? Both laws prevent local enforcement officers from asking for a person’s immigration status, detaining someone because of their immigration status or cooperating with federal immigration agents in several crucial areas.This helps protect undocumented immigrants because ICE agents seek local law enforcement help when they’re going after noncitizens with reported immigration violations. When an undocumented person is incarcerated, for example, ICE might issue a detainer asking local law enforcement to notify them before releasing these noncitizens and to hold them an additional 48 hours until ICE can take custody. Illinois law prohibits this type of collaboration, except where the person faces a federal criminal warrant. Chicago makes the same exception.Illinois law also prevents local officials from supporting ICE with manpower, such as transporting detained immigrants or creating security perimeters for immigration raids.Law enforcement officials throughout the state and any government authorities in Chicago are also prohibited from sharing information about undocumented immigrants, including their addresses. This is particularly important to DACA recipients or to immigrants who have secured Temporary Protected Status to be in the country legally and who fear they could become targets if Trump ends those programs.Both sanctuary laws also prohibit local officials from asking an individual’s immigration status during routine interactions, such as traffic stops.Why were sanctuary policies created?Experts say sanctuary laws help protect undocumented immigrants from being criminalized and targeted by local police. In turn, they help build trust between police and local communities, and research shows this reduces crime.Chicago’s sanctuary city status dates back nearly 40 years, when then-Mayor Harold Washington issued an executive order prohibiting city employers from asking about an individual’s immigration status or assisting federal immigration officials with enforcement. In 2006, the order became city law as the national immigration debate took center stage in Congress. In 2012, this law was expanded and codified as the Welcoming City ordinance. In 2017, former Illinois Gov. Bruce Rauner signed the TRUST Act, prohibiting law enforcement agents throughout the state from collaborating with ICE.Can local law enforcement stop federal agents from arresting undocumented immigrants?The federal government can enforce immigration law anywhere in the country, even if local authorities don’t help.So, for example, city and state law enforcement agents can’t stop ICE agents from arresting noncitizens if the agents have a warrant signed by a judge ordering the arrest and deportation of an individual.Trump recently confirmed plans to declare a national emergency and use the U.S. military to assist with the mass deportation of undocumented immigrants. While such an action would violate federal law under normal circumstances, experts have speculated that Trump could invoke the Insurrection Act of 1807, which gives the president emergency powers to use federal troops on domestic soil to curb local unrest or rebellion.Such a decision would likely face legal challenges."The Trump administration would have to argue that the mere presence of undocumented people or immigrants in a community rises to that level," said Fred Tsao, senior policy counsel at the Illinois Coalition for Immigrant and Refugee Rights. "Any fair-minded federal official would have trouble squaring with the spirit of the law."But "the federal courts are very different from the way they were four years ago [or] eight years ago," he added. With Republicans controlling Congress next year, "Trump is going to have an easier time confirming new judges. So, that’s a concern."