Olney v. Arnold was a landmark case in the early history of the United States Supreme Court, which established that state courts must follow federal law when deciding cases involving federal questions. The dispute arose from a contract between two merchants, Olney and Arnold, to buy goods for resale at an agreed-upon price. When Olney failed to pay on time, Arnold sued him in Pennsylvania state court for breach of contract and won a judgment against him. Olney appealed the decision to the U.S Supreme Court arguing that he had been denied due process under Article III of the Constitution because his rights were not adequately protected by Pennsylvania's laws regarding contracts with foreign merchants like himself who lived outside its jurisdiction. In ruling in favor of Olney, Chief Justice John Jay held that it was unconstitutional for states to deny individuals their constitutional right to due process when they are involved in disputes over matters arising out of federal law or treaties made under authority granted by Congress; thus establishing precedent requiring all state courts adjudicating such matters must abide by applicable provisions set forth within those laws or treaties as well as any other relevant decisions handed down from higher judicial authorities including those issued by US Supreme Court itself

In Olney v. Arnold, the Supreme Court held that a state court could not issue an injunction to prevent a federal court from exercising its jurisdiction over a case. Chief Justice John Jay wrote in dissent that he believed the decision was contrary to both constitutional and statutory law. He argued that it would be wrong for one branch of government to interfere with another's exercise of power, as this would undermine the separation of powers established by the Constitution and lead to confusion and conflict between branches. Furthermore, he noted that Congress had specifically authorized state courts to enjoin proceedings in federal courts when necessary or proper under certain circumstances; thus, denying such authority here ran counter to congressional intent. Ultimately, Jay concluded his opinion by stating his belief that allowing states' interference with federal judicial proceedings violated principles essential for maintaining our nation’s system of checks and balances among different branches of government