Significance: Known as one of Abraham Lincoln's best friends, Davis is best known for writing the opinion in an 1866 Supreme Court case that limited the use of military authority over civilians in areas not threatened by military action.
Upon graduation from Kenyon College in Ohio in 1832, Davis moved to Massachusetts and read law with a local judge, Henry W. Bishop. Davis attended Yale Law School and was admitted to the Illinois bar in 1835, establishing a private practice in Pekin. During this time, Davis met Abraham Lincoln, a member of the Illinois legislature, who became a lifelong friend. In 1836 Davis moved to Bloomington, Illinois, where he steadily built a reputable law practice. In 1844 Davis won a seat in the Illinois legislature and served on the education committee. As an elected member of the Illinois Constitutional Convention in 1847, Davis was instrumental in changing the judicial system by advancing reforms so that judges were elected by the people instead of by the legislature. In 1848 Davis was elected as a circuit judge on the Illinois Eighth Circuit, a position he held for fourteen years. Both Lincoln and Stephen Douglas tried cases in his court. Davis campaigned for Lincoln in his two losing bids for the U.S. Senate. At the 1860 Republican convention, Davis orchestrated the nomination of Lincoln for president of the United States. Subsequently, he advised Lincoln on campaign strategy. After Lincoln's election, Davis advised him in assembling the cabinet. In 1862 President Lincoln nominated Davis to the Supreme Court. Davis never demonstrated a strong interest in legal scholarship and wrote few important Court opinions. His most noteworthy decision came in Ex parte Milligan (1866). The case involved a civilian, Lambdin P. Milligan, who was tried by an Indiana military court and convicted of conspiracy during the Civil War. Eventually, Milligan appealed his case to the Supreme Court. Davis argued that constitutional rights do not cease to exist during wartime and that Milligan's rights had been violated when he was tried by a court that was not sanctioned by Congress and also when he was denied his right to trial by a jury. Davis concluded that the president of the United States had no power to mandate the trial of civilians by a military commission in areas where civilian courts were operating. After serving on the Court for fourteen years, Davis became bored and wanted to return to the excitement of the political arena. In 1872 Davis had been an unsuccessful candidate for nomination for the president of the United States on the Liberal-Republican ticket. In 1877 he resigned his position on the Court when he was elected as a U.S. senator from Illinois. From 1881 to 1883 Davis served as president pro tempore of the Senate.