President Donald Trump s plan to stop his administrations policy of separating immigrant children from their families seems simple: Lock up immigrant parents and their children together, indefinitely.

But theres no evidence that Trump has the legal authority to make his wish reality. His plan, issued in an executive order on Wednesday, conflicts with a 2015 court ruling that required the government to release child migrants from detention after 20 days. Trump cant dismiss federal judges rulings by decree. So his executive order will trigger a massive showdown between his administration and human rights activists in court.

These are protections for kids. Thats the doctrine here, said Muzaffar Chishti, director of the nonpartisan Migration Policy Institutes office at the NYU School of Law. And if anything goes against the protection of kids, it will be challenged.

Trump administration officials have insisted that they enacted the family separation policy in part because of a 2015 court order over a 1997 court settlement called the Flores agreement. The Flores agreement and the ensuing 2015 ruling limit the length of time that children can be detained, including with their parents. Most detained immigrant children must be released within about 20 days to continue their deportation proceedings outside of detention. In the past, the federal government has generally released the childrens mothers from family detention along with them rather than splitting them up.

The Trump administration has said this policy effectively creates catch and release for families, and proposed changing it through legislation. But getting immigration policy through Congress is an uphill battle.