United States v. Klein was a United States Supreme Court case that addressed the issue of whether the federal government could use a private party to enforce a federal law. The case involved a claim by the United States against a private party, Klein, for the return of a bounty paid to him by the federal government for enlisting in the Union Army during the Civil War. The Court held that the federal government could not use a private party to enforce a federal law, and that the government must seek its remedy through the courts. The case arose when Klein was paid a bounty for enlisting in the Union Army during the Civil War. After the war, the federal government sought to recover the bounty, claiming that Klein had fraudulently obtained it. Klein argued that the government could not use a private party to enforce a federal law, and that the government must seek its remedy through the courts. The Supreme Court agreed with Klein, holding that the federal government could not use a private party to enforce a federal law. The Court reasoned that the government must seek its remedy through the courts, as the Constitution does not grant the government the power to use a private party to enforce a federal law. The decision in United States v. Klein established an important precedent in the area of federal law enforcement. The Court held that the federal government must seek its remedy through the courts, and cannot use a private party to enforce a federal law. This decision has been cited in numerous subsequent cases, and has been an important factor in the development of federal law enforcement.

In United States v. Klein, the Supreme Court was tasked with determining whether a pardon issued by President Andrew Johnson to an individual who had been convicted of aiding and abetting the Confederate cause during the Civil War could be used as evidence in a tax case against him. The majority opinion held that it could not, since pardons are considered acts of grace from the president which do not necessarily imply innocence or guilt on behalf of those receiving them. In his dissent, Justice Field argued that while pardons may indeed be acts of grace from the president, they should also carry some legal weight when presented as evidence in court cases such as this one. He further stated that if Congress had intended for pardons to have no legal effect whatsoever then there would have been no need for them at all; thus he concluded that allowing their use in certain circumstances is both reasonable and necessary under existing law.