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Abraham Lincoln's Invention of Presidential War Powers: Facing the unprecedented crisis of civil war in 1861, President Abraham Lincoln invoked his "war power" as commander-in-chief to "take any measure which may best subdue the enemy." Defying the chief justice of the United States, he suspended the writ of habeas corpus by presidential decree. He also declared martial law, authorized the trial of civilians by military courts, and proclaimed the emancipation of slaves--all on the grounds that "I may in an emergency do things on military grounds which cannot be done constitutionally by Congress." In so doing, Lincoln vastly expanded presidential war powers and established precedents invoked by later presidents.

Sauce: https://jmp.princeton.edu/events/abraham-lincolns-invention-presidential-war-powers

"Abraham Lincoln's Invention of Presidential War Powers: Facing the unprecedented crisis of civil war in 1861, President Abraham Lincoln invoked his "war power" as commander-in-chief to "take any measure which may best subdue the enemy." Defying the chief justice of the United States, he suspended the writ of habeas corpus by presidential decree. He also declared martial law, authorized the trial of civilians by military courts, and proclaimed the emancipation of slaves--all on the grounds that "I may in an emergency do things on military grounds which cannot be done constitutionally by Congress." In so doing, Lincoln vastly expanded presidential war powers and established precedents invoked by later presidents."