Abraham Lincoln declared an emergency when congress was not in session, called up troops, suspended habeas corpus, and initiated trials by military commission rather than conventional civil courts. When congress came back they passed legislation to support his actions.

https://www.civiced.org/lincoln-lesson

Suspension of habeas corpus means they can round up people and detain them indefinitely without bringing charges. Suspension of habeas corpus does not automatically mean a military tribunal. In fact, the supreme court ruled that so long as civil courts were functioning, civilians cannot be tried by the military. (Ex parte Milligan, 71 U.S. (4 Wall.) 2 (1866))

https://en.m.wikipedia.org/wiki/Ex_parte_Milligan