Ward v. Maryland was a landmark Supreme Court case that established the principle that a state cannot pass a law that interferes with the rights of citizens under the U.S. Constitution. The case arose when a Maryland law was passed that required merchants to post a bond in order to do business in the state. The plaintiff, John Ward, was a merchant who refused to post the bond and was subsequently arrested. Ward argued that the law was unconstitutional because it violated his right to due process and equal protection under the law. The Supreme Court agreed with Ward and ruled that the law was unconstitutional. The Court held that the law was an unconstitutional interference with the rights of citizens under the U.S. Constitution. The Court reasoned that the law was an arbitrary exercise of power by the state and that it violated the due process and equal protection clauses of the Fourteenth Amendment. The decision in Ward v. Maryland was a major victory for civil rights and established the principle that a state cannot pass a law that interferes with the rights of citizens under the U.S. Constitution. The decision has been cited in numerous cases since then and has been used to protect the rights of citizens from arbitrary state laws.

In Ward v. Maryland, the Supreme Court was asked to decide whether a state law that required merchants and traders to obtain a license in order to do business within its borders violated the Fourteenth Amendment's Privileges or Immunities Clause. The majority opinion held that it did not, as the right of an individual to pursue any lawful employment is not one of those privileges or immunities protected by this clause. However, Justice Field dissented from this decision on two grounds: firstly, he argued that under Article IV Section 2 of the Constitution (the Privileges and Immunities Clause), citizens are guaranteed certain rights when they enter another state; secondly, he argued that even if these rights were not explicitly stated in Article IV Section 2, they should still be considered fundamental rights which cannot be abridged without due process of law. In conclusion, Justice Field believed that states should not have absolute power over their citizens' economic activities but rather must respect their constitutional protections against arbitrary interference with such activities.