In Davis v. United States (1896), the U.S. Supreme Court ruled that a military officer could not be tried in civilian court for actions taken while performing his official duties, even if those actions were later deemed illegal. The case involved Lieutenant Colonel Jefferson C. Davis who was charged with murder after shooting and killing General William Nelson during an argument in 1862, amidst the Civil War chaos. Despite being acquitted by a military tribunal, he was subsequently indicted by a Kentucky grand jury when the war ended but never brought to trial due to jurisdictional disputes between civil and military authorities. The Supreme Court held that since Lt.Colonel Davis' act occurred within a theater of war while he was acting under orders from superior officers as part of his official duties, it fell under exclusive federal jurisdiction via martial law rather than state authority or common law rules governing homicide cases generally applicable outside wartime conditions. This ruling established important precedents regarding immunity for armed forces personnel from prosecution in civilian courts for acts committed during active service and clarified boundaries between civil and military jurisdictions over such matters.

In the dissenting opinion for Davis v. United States (1896), Justice Harlan argued that the majority's decision was a departure from established principles of international law and violated U.S. treaty obligations to China. He contended that Chinese immigrants, like all other people residing in America, were entitled to protection under American laws and should not be subjected to discriminatory treatment based on their nationality or race. Furthermore, he asserted that Congress did not have unlimited power over immigration policy and could not enact laws infringing upon individual rights guaranteed by the Constitution. In his view, the Geary Act - which required Chinese residents in America to obtain certificates of residence or face deportation - was unconstitutional as it imposed an unreasonable burden on these individuals without due process of law.