President Donald Trump launched his sweeping immigration crackdown on Monday which included an order reinterpreting birthright citizenship, a principle that has been recognized in the United States for more than 150 years.

Trump and his allies have claimed that birthright citizenship encourages what they call “birth tourism” – claiming that people deliberately travel to the US when pregnant in order to give birth there, gaining citizenship for their child and also giving them a stronger chance of gaining residency via their child.

President Trump’s public rhetoric has focused on undocumented immigrants, but the raft of new orders he signed would also affect those seeking to enter the U.S. legally.