U.S. Secretary of Defense James Mattis resigned from his position on Thursday, saying that the position should go to someone whose views are more closely aligned with those of U.S. President Donald Trump.

The move comes following a sudden announcement from Trump on Wednesday that the U.S. would withdraw all forces from Syria.

In his resignation letter to Trump, Mattis said the president had “a right to have a Secretary of Defense whose views are better aligned with” his own.

Mattis also cited the importance of Washington’s alliances and partnerships, while warning of the dangers presented by countries like China