Final answer:
John Marshall's significant accomplishment as Chief Justice was establishing the power of judicial review in the case of Marbury v. Madison in 1803. This granted the Supreme Court the authority to invalidate congressional and presidential actions if deemed unconstitutional, profoundly influencing the balance of power in the U.S. government.
Step-by-step explanation:
John Marshall's Important Accomplishment as Chief Justice
One of John Marshall's most important accomplishments as Chief Justice of the Supreme Court was establishing the power of judicial review. This was established through the landmark case Marbury v. Madison in 1803. In this case, Marshall found that Congress's Judicial Act of 1789 was unconstitutional as it allowed for cases to come directly before the Supreme Court, which was not permitted by the Constitution itself. Consequently, Marshall asserted the Supreme Court's power to review and potentially nullify actions of Congress and the president, which is known today as judicial review. This has since become a central aspect of the court's ability to check the other branches of government and the states.
Under Marshall's leadership, the Supreme Court clarified its role in the government and significantly influenced national policy and public opinion. The decision in Marbury v. Madison reinforced the influence and independence of the judiciary, and it is considered one of the defining moments in U.S. legal history.