You’re not imagining it: Sanctuary cities really have reached peak media saturation in recent years. But what does it all mean? In our recently released book, Sanctuary Cities: The Politics of Refuge, we explore the politics and effects of sanctuary city policies, which became a prominent campaign issue in 2016 and are likely to feature in the upcoming 2020 presidential campaign.

Though they’ve become a media fixation of late, sanctuary cities have been around since the mid-1980s. We define the term as “a city or police department that has passed a resolution or ordinance expressly forbidding city or law enforcement officials (i.e., police) from inquiring into individuals’ immigration status.” Such cities often refuse to cooperate with Immigration and Customs Enforcement (ICE). These policies are designed to encourage greater police cooperation among undocumented residents or are implemented in opposition to what local lawmakers deem aggressive federal immigration enforcement. Immigration advocates argue that these policies benefit local communities because undocumented individuals who fear deportation may not call 911 if they are a witness to or a victim of a crime.

Sanctuary cities originate from the Sanctuary Movement of the 1980s when civil conflicts erupted in Guatemala and El Salvador, displacing hundreds of thousands of civilians. Many refugees fled to the United States seeking political asylum. However, the U.S. disproportionately denied these Central Americans asylum. In response, churches and synagogues across the U.S. started housing refugees, citing religious tradition. Not long after, some cities passed resolutions and ordinances designed to provide Guatemalans and Salvadorans safe harbor within the city and to show support for their local churches offering refuge to Central Americans.

More than a decade later, new sanctuary cities aiming to protect undocumented immigrants sprung up in response to the 9/11 security crackdown ushered in by the Bush administration. The Bush administration, and later the Obama administration, dramatically increased spending on immigration enforcement, leading to an unprecedented increase in detention and deportation of the undocumented population. So politicians favoring strict enforcement of immigration law came to see sanctuary as a liberal policy protecting communities of people who are not supposed to be in the United States. Sanctuary proponents pushed back, saying the disruption of such communities through deportations caused severe harm to citizens and non-citizens alike. Additionally, many argued that sanctuary policies help local law enforcement by increasing trust between immigrant communities and the police.

For the most part, though, sanctuary policy wasn’t a major topic of national discussion until the early days of the 2016 presidential campaign. In July 2015, a young woman, Kathryn Steinle, was accidentally shot in San Francisco (a sanctuary city) by an undocumented immigrant, leading to a series of claims by then-candidate Donald Trump and other top Republicans that sanctuary city policies caused or increased crime and should be banned.

A broad Democratic defense did emerge after Trump was elected, with then-Chicago mayor Rahm Emanuel saying shortly after the 2016 election: “Chicago has in the past been a sanctuary city… It always will be a sanctuary city.” When Trump took office, his administration promptly threatened to cut federal funds to such cities if they did not revoke their sanctuary status. Philadelphia, Seattle, New York, and other major cities fought back. However, some places like Miami-Dade County later conceded to the new administration by retracting their sanctuary policy.

During the first year of the Trump administration, California passed a law making the whole state a sanctuary, while Texas banned sanctuary cities outright. The number of bills introduced into state legislatures related to sanctuary cities (either pro or con) increased from 11 in 2016 to about 150 in 2017.