States have often been referred to as sister states. States must interact harmoniously as equals, with none more important than any other notwithstanding differences in size and resources. The Constitution provides a structure for voluntary cooperation and sometimes rules for forced cooperation among states.
The explicit grants of power to Congress and limitations on states in Article I effectively limit how states can act toward one another. One example of these limitations is the power given to Congress to regulate interstate commerce. Over time, congressional regulation of commerce has preempted much of the mischief that states attempted to do to other states over differences in resources. In the same vein, Article I limits Congress from privileging some states over others with respect to the regulation of goods traveling through ports by guaranteeing some level of commercial equality among states. Of course, states may cooperate voluntarily with each other in strengthening their commercial relationships.
The Constitution also names explicit requirements for promoting cooperation among the states. Two examples illustrate this forced cooperation. Article IV of the 1787 Constitution prohibited the states from interfering with the return of fugitive slaves to their masters. When two slave states were involved, the requirement was a matter of cooperation. However, as states began to abolish slavery, the prohibition that free states could not protect runaway slaves from recapture by their masters became a matter of conscience for some. Nonetheless, until slavery was abolished, the forced cooperation remained in place over the objections of the abolitionists.
Article IV provides an additional mechanism for forced cooperation—the Full Faith and Credit Clause. A holdover from the Articles of Confederation, the Full Faith and Credit Clause guarantees that “the public acts, records, and judicial proceedings” of one state will be honored in all other states. The clause is an aid to interstate travel and guarantees that the official acts of one state will not be a dead letter in another state. For example, the Full Faith and Credit Clause guarantees that a court judgment from one state almost always will be enforceable in another state. That has two implications. The first is that one state may be required to allow access to its courts regarding a matter that did not occur in that state. The second is that a person may not evade civil justice from one state by simply moving to another state. A person also may not evade criminal justice, because a different section of Article IV requires that fugitives from justice be returned to that state from which they fled.