Congress's aversion to its duty.

Rep. Barbara Lee (D-CA) and other legislators have filed amendments to the 2017 National Defense Authorization Act (NDAA) in an effort to force a broad congressional debate, especially related to the war against the Islamic State (ISIS). It’s not simply that Congress hasn’t debated the use of force in the war against ISIS. It’s that as a body they actively resist such debate.^[1]

The Republican Congress, that is.

Notes
^[1] "Congress Is Ignoring Its Most Important Constitutional Duty." By Daniel L. Davis, The National Interest, 5/17/16.