[Dalewyn]

It's worth noting that in the US government hierarchy, the United States Congress is superior to the Department of Defense. Why? Because they (both House and Senate) write the laws stating who and what the DoD is, the House funds the DoD, and the Senate confirms appointments of officers.

Specifically with regards to Matt Rosendale, he is a Representative from Montana. He is in a position where he can demand (and will get, within reason) information from the US military and represent Montana in demanding and publicizing said information.

[hedora]

That is not how the chain of command works for the military. The president is the commander in chief, and senators do not report to him.

[wyldfire]

Ultimately their paramount allegiance is to the Constitution and the people of the United States. This can supercede the chain of command when Commander-in-chief places orders contravened by the People.

Congress performs oversight of the military and sets budgets, so they're briefed on operations like this which have some outsized political impact. It doesn't seem to me like these things should be a big deal but the rest of the people in the US seem to act like it is, so Congress is naturally very interested.