Alexander v. Roulet was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The case arose when a prisoner, Alexander, was held in a federal prison in Louisiana. Alexander sought a writ of habeas corpus from the state court, which the court granted. The federal government then appealed the decision to the Supreme Court. The Supreme Court held that the state court did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. The Court reasoned that the writ of habeas corpus was a federal power, and that the state court did not have the authority to interfere with the federal government's power to imprison individuals. The Court also noted that the writ of habeas corpus was a fundamental right, and that the state court should not be allowed to interfere with the federal government's power to protect this right. The decision in Alexander v. Roulet was significant because it established that the state courts did not have the authority to issue a writ of habeas corpus to a prisoner held in a federal prison. This decision has been cited in numerous cases since then, and it has been used to support the idea that the federal government has the power to protect the fundamental right of habeas corpus.

In Alexander v. Roulet, the Supreme Court was asked to determine whether a contract between two parties could be enforced if it had been made in violation of state law. The majority opinion held that such contracts were not enforceable and thus the plaintiff's claim should be dismissed. However, Justice Field dissented from this decision and argued that while states have the right to regulate commerce within their borders, they cannot interfere with private contracts which are otherwise valid under federal law. He further noted that enforcing these types of agreements would encourage people to abide by laws rather than violate them for personal gain or advantage. Ultimately, he concluded that allowing individuals to make binding agreements even when those agreements may conflict with state statutes is essential for protecting individual rights and promoting economic growth throughout the nation as a whole.