UPDATE: All clear. The issues have been resolved, and servers should be operational once again.
Activision has updated the status page for Call of Duty: Black Ops 2, Advanced Warfare, and Black Ops 3 to state that the servers are currently at DEFCON 1 with services offline. Many users are reporting connection issues.
Activision has stated that their operations team is aware and working on fixing the issue as soon as possible.