President Donald Trump’s executive order denying U.S. citizenship to the children of parents living in the country illegally has faced the first of what will be many legal tests. It didn’t fare well.

A federal judge Thursday blocked President Donald Trump’s executive order outlawing birthright citizenship —the policy automatically giving citizenship to anyone born in the U.S.—and declared it “blatantly unconstitutional,” as legal experts widely believe the president does not have the power to change the policy on his own.

One of the many executive orders President Donald Trump signed on Monday included one to end birthright citizenship. Here's what to know about it and why lawsuits have already been filed.