Trump signed an Executive Order to end Birthright Citizenship, where a person born in the United States automatically becomes a citizen, as is mandated in the US Constitution.
Some of the most amazing people in US history, who have made massive contributions to our nation, were born to immigrant parents and received citizenship this way. Ending Birthright Citizenship will prevent us all from benefiting from many future people of this caliber.
It may be difficult for Trump to actually accomplish this, given it is blatantly unconstitutional, so we are Watching this for now.
Update: Trump is appealing to the Supreme Court to give him the power to violate the Constitution and get rid of Birthright Citizenship.
No prior administration in at least the last century has ever made a serious effort to undo this provision of the US Constitution. What more is there to say on this?