President Trump has become the third president over the course of 16 consecutive years to extend the "temporary state of national emergency" which was imposed in the wake of the 9/11 terror attacks.
The state of emergency gives the president considerable leeway to bypass legal limits surrounding the military and the use of the National Guard and reserve troops. Exactly how much this power is used is itself unclear, as presidents have for years ignored a legal requirement to offer reports to Congress every six months on the matter.
Officially, Presidents Bush, Obama, and now Trump have defended the extension on the grounds that terrorism still exists and is a thing. Legally speaking, however, the emergency declaration was intended to give the president flexibility until Congress had time to act after 9/11.