Article IV of the Constitution forces states to cooperate with each other rather than operate as rival nations. The Full Faith and Credit Clause requires every state to honor other states' court judgments—a divorce finalized in Nevada, for example, must be recognized in Texas.
The Privileges and Immunities Clause (sometimes called the "comity clause") bars states from discriminating against visitors in favor of their own residents on fundamental rights like earning a living, owning property, or accessing courts. And the Extradition Clause requires states to return criminal fugitives to the state where they face charges.
These provisions solve a practical problem the Founders understood well: without binding rules, states could refuse to enforce each other's contracts, lock out-of-state workers out of professions, or harbor fugitives. The Supreme Court has ruled that some distinctions between residents and nonresidents are permissible—like charging higher tuition at state universities—but only when they don't threaten the functioning of the country as "a single Union." Interstate comity is the constitutional glue that keeps 50 separate legal systems from working against each other.