I believe the President has a constitutional responsibility to defend our country against all enemies, foreign and domestic, and things such as violent foreign gangs killing our own citizens, massive amounts of drugs destroying families and entire communities, etc. constitutes a national emergency that it is one of the President’s foremost responsibilities to protect us against.
Article IV, Section 4, so yeah - Constitutional duty. Not belief - fact. “The United States shall...”; applies to Executive, Congress, Judiciary, and the States themselves in relation to the other States
Oath of Office, also.