President-elect Donald Trump was surprisingly gracious as he met President Barack Obama in the Oval Office on Thursday but make no mistake: He is already working to erase major parts of Obama’s legacy from the history books.

Trump will be able to change some of Obama’s policies with a quick stroke of the pen. Others will be much more difficult, requiring justification to pass legal hurdles or buy-in from lawmakers on Capitol Hill or foreign leaders.

“He can make a big difference at the outset of his administration, but it will take him years and support from dubious congressional factions and allies overseas to get a lot of other things done,” said Charles Tiefer, former solicitor and deputy general counsel of the House of Representatives and now a professor at the University of Baltimore law school.

Despite the strong rhetoric on the campaign trail, Obama and Trump were complimentary of each other after meeting for 90 minutes — longer than expected — seated alone in the Oval Office Thursday.

“I have been very encouraged by the, I think, interest in President-elect Trump’s wanting to work with my team around many of the issues that this great country faces,” Obama told reporters after the meeting.

It was the first time the two had met. Obama described the conversation as “excellent,” while Trump called Obama “a very good man.”