Pico v. United States was a Supreme Court case in which the court ruled that Congress had no authority to pass legislation that would interfere with state laws concerning slavery. The case arose from an act of Congress passed in 1862, which prohibited any person or persons from holding slaves within the limits of any State or Territory then at war with the United States. Pico argued that this law violated his rights as a citizen and property owner under California's Constitution, since it interfered with his right to own slaves under California law. The Supreme Court agreed and held that Congress did not have the power to pass such legislation because it was beyond its constitutional powers granted by Article I Section 8 of the US Constitution, which only gave Congress limited powers over interstate commerce and foreign affairs; thus, they could not regulate domestic issues like slavery without violating states' rights protected by the Tenth Amendment. This decision reaffirmed states’ rights against federal encroachment on matters traditionally left for individual states to decide upon their own discretion

In Pico v. United States, the Supreme Court was asked to decide whether Congress had the power to pass a law that would allow for the confiscation of property belonging to individuals who supported or sympathized with Confederate forces during the Civil War. The majority opinion held that Congress did have such authority under its war powers and could thus lawfully enact legislation allowing for such confiscations. However, Justice Field dissented from this decision and argued that while it is true that Congress has broad powers in times of war, these should not be interpreted so broadly as to include an act which deprives citizens of their private property without due process of law. He further noted that even if there were some legitimate purpose behind such a measure, it still violated fundamental principles enshrined in both state constitutions and federal laws which protect individual rights against arbitrary government action. In conclusion he stated his belief that any attempt by Congress to take away private property without due process was unconstitutional and therefore invalidated by this court's ruling on Pico v United States