Ex Parte Milligan was a landmark Supreme Court case that established the limits of military tribunals and martial law. The case centered around Lambdin P. Milligan, an Indiana resident who had been arrested by the Union Army during the Civil War for conspiring to aid Confederate forces and plotting against government officials in Washington D.C.. He was tried before a military commission rather than a civil court, which he argued violated his constitutional rights as a citizen of Indiana not engaged in rebellion against the United States. In its ruling, the Supreme Court held that citizens cannot be tried by military commissions unless they are actively engaging in war or insurrection against their own country; otherwise they must be afforded all due process protections under civilian courts as guaranteed by both state and federal constitutions. This decision has since become one of our nation's most important legal precedents regarding civil liberties during times of conflict or emergency situations when martial law is declared.

In Ex Parte Milligan, the Supreme Court held that a military tribunal could not try civilians in areas where civil courts were open and functioning. The dissenting opinion argued that Congress had the power to suspend habeas corpus during times of rebellion or invasion when public safety required it. Furthermore, they argued that since there was no proof of any conspiracy against the government by Lambdin P. Milligan, he should have been tried before a jury as provided for under Article III of the Constitution rather than by a military court-martial. They also noted that even if Congress had suspended habeas corpus, this did not mean it could authorize trials without juries or deny individuals their right to be heard in an appropriate forum such as a civilian court with all its attendant protections and safeguards. Finally, they asserted that while martial law may be necessary at certain times to protect citizens from danger posed by rebels or invaders, it must always remain subject to constitutional limitations on governmental powers so as not to infringe upon individual rights guaranteed under our system of laws and justice.