United States v. Windsor
The court decided that defining "marriage" as a union between one man and one woman (as husband and wife) was unconstitutional (against the Constitution). This specifically related to Section 3 of the Defense of Marriage Act (DOMA), a law passed in 1996. This defined "marriage" for federal (national) law in the United States. It defined it as being between one man and one woman. In United States v. Windsor, the court decided that this definition was against the Due Process Clause of the Fifth Amendment. In the U.S., the Constitution protects the rights of citizens, and no individual law can take those rights away.