The US Defense Secretary, James Mattis is resigning and will leave the job in February.
This announcement came a day after President Trump said he was withdrawing troops from Syria, a decision James Mattis is understood to oppose.
President Trump has not immediately named a successor but one would be appointed shortly.
Members of Congress from both sides of the political divide have reacted with shock to the resignation.
Gen. Mattis implied to variances with the president in a number of policy areas.
Gen. Mattis had positioned himself as one of the cool heads throughout the president’s term.