The State Department was meant to be the counterpart to the War Department, focusing on diplomacy and peace. What ended up happening as the US government morphed into a globe-spanning empire dependent on endless violence and aggression is that the State Department wound up focusing more and more on manufacturing interventionist narratives on the world stage to gin up international support for starvation sanctions, proxy wars and war coalitions.

So in practice the US ended up with two war departments: the DoD and the State Department. Which is why you’ve seen the nation’s Secretaries of State becoming more and more jingoistic and psychopathic, to the point where some sort of antisocial personality disorder is almost a job requirement for the position.