The case of Cohens v. Virginia was decided in 1821. It was a unanimous decision by the Supreme Court which was led at that time by the famous chief justice John Marshall. This was an important case because, like many cases that Marshall decided, it increased the power of the national government.
When the Constitution was created, it did not clearly specify the relative powers of the federal and state governments. For example, it did not explicitly say that the federal government could overrule a state law if it felt that the law was contrary to the Constitution. This became an important issue in the new country.
In 1821, the Court decided Cohens. In that case, it decided that the federal government (specifically the Supreme Court) could overrule a criminal law enacted by a state if it felt that the law was unconstitutional. In this case, a man bought lottery tickets in Washington, D.C. where a lottery was legal, and resold them in Virginia, where such an action was illegal. The government of Virginia tried and convicted the man, saying that it had the right to decide whether state law trumped national law. The Supreme Court took the case and overruled the Virginia court. The Supreme Court held that it had the right to review the decision of a state court whenever a constitutional question was involved. This was significant because it gave the federal government (in the form of the Supreme Court) more power than it had previously had.