As the fourth chief justice, John Marshall is credited with shaping the American system of constitutional law. After the Revolutionary War, Marshall returned to his Virginia home to study and began to practice law. He became active in state politics, serving in the House of Delegates, and emerged as a political rival to his distant relative, Thomas Jefferson. Marshall was named chief justice by John Adams two months before the end of his presidency.
The important Marbury v. Madison case, in Marshall's court, was the first in which the court declared an act of Congress unconstitutional. Thus was established the concept of judicial review by the federal courts over acts of the two other branches of government. Among other major cases before his court, McCulloch v. Maryland prohibited states from taxing a federal instrument, the Bank of the United States. In Cohens v. Virginia and Gibbons v. Ogden, Marshall wrote opinions that established the unity of the federal court system and set the precedent for the assertion of federal authority over the states when their individual interests clashed.
In 1807 Marshall presided over the treason trial of Aaron Burr. Against the wishes of Thomas Jefferson, Burr was found innocent. During Marshall's 35 years as chief justice, he took part in more than 1,000 decisions, of which he wrote 519. He also wrote a five-volume Life of George Washington.