To understand why, one needs to understand exactly what sanctuary laws do. They draw their name from the sanctuary movement of the 1980s. During that period, Central American refugees were routinely denied asylum because the United States government supported the regimes from which they had fled. In the face of this injustice, some religious leaders took steps to actively prevent federal immigration officers from arresting and deporting these vulnerable immigrants. It was, at times, a form of civil disobedience.

Today’s sanctuary laws, while bearing the same name, are markedly different. California and the hundreds of other places across the country with such laws and policies have done nothing whatsoever to actively interfere with federal immigration enforcement efforts. Rather, the defining characteristic of these laws is their passivity. They do not direct state officers to take any steps to interfere with federal enforcement efforts. Instead, they dictate that the local police and state officers simply do not assist in the federal government’s deportation agenda — that they do nothing.

This may sound like an overly formal distinction, since both efforts have made it harder for the federal government to deport people. To be sure, the decision of local and state officials not to lend their resources to immigration enforcement efforts means that either there will be fewer deportations or the deportations will be more expensive for the federal government. This is one reason these laws have so rankled Mr. Trump and Mr. Sessions.

The Justice Department lawsuit emphasizes that immigration is a federal matter, that we must have a uniform scheme to oversee it and that this scheme is being undermined by sanctuary laws. In most states, federal immigration authorities are able to leverage state and local criminal justice systems. The Justice Department is arguing that California’s refusal to participate requires it to adapt and employ different enforcement strategies.