What is the incorporation doctrine?

For nearly a century after ratification, the Bill of Rights restrained only the federal government, not the states. Barron v. Baltimore (1833) made that explicit: Chief Justice Marshall held the Fifth Amendment's Takings Clause didn't apply to a city government, because the first ten amendments were written as limits on federal, not state, power. That changed gradually through a doctrine called incorporation, built on the Fourteenth Amendment (1868), which bars states from depriving any person of “life, liberty, or property, without due process of law.” The Supreme Court read that Due Process Clause as absorbing — incorporating — specific Bill of Rights protections one at a time, making them binding on state governments too. Gitlow v. New York (1925) is usually marked as the turning point: the Court assumed, for the first time, that the First Amendment's free speech and free press protections applied to the states through the Fourteenth Amendment, even though it upheld Gitlow's conviction on the facts of the case. From there, the Court incorporated other rights case by case rather than all at once — this piecemeal approach is called “selective incorporation.” Most, but not all, of the Bill of Rights has since been applied to the states this way; the Third Amendment's protection against quartering troops and the Fifth Amendment's grand jury requirement are among the few provisions that still haven't been formally incorporated. The practical effect is large: without incorporation, a state legislature could ban a religion, censor a newspaper, or search a home without a warrant, and the federal Bill of Rights alone would offer no protection against it.