An agreement between the FAA and Department of Defence (DoD), this has apparently been in the works for several months and is not in response to a specific threat.
Civilian drones, often armed in some fashion, have been used in combat zones, with varying degrees of success. Meanwhile the number of privately owned drones in the US now outnumbers military drones by a factor of at least twenty, creating a unique and new dynamic.
The specific restricted locations that are affected by the FAA’s restrictions are detailed in a Notice to Airmen: UAS NOTAm FDC 7/7137. The FAA has the authority to establish these restrictions under Title 14 of the Code of Federal Regulations, (14 CFR) § 99.7 – Special Security Instructions. Relief provided under this is limited to requests from the Department of Defense and U.S. federal security and intelligence agencies based on national security interests. Military facilities are deemed “sensitive” due to being vital to national security.
Drone flights up to 400 feet within any of these facilities’ lateral boundaries are restricted, with only a few exceptions that will permit drone flights, to be arranged directly with the facility or with the FAA. Violators of the airspace restrictions may be liable to potential civil penalties and criminal charges.
These restrictions take effect from 14th April 2017.
References: sUAS News FAA Interactive map