Our national history has presented various phases of this problem. Slavery showed one; the complexities of interstate commerce have kept others constantly in mind. On the day the Constitution was finished, probably more people would have seen seeds of conflict and dangers to the Union in future commercial relations than in slavery. But commerce became a source of strength, while slavery be- came a cause of division. It brought the Union into danger; and in the end was destroyed itself. Where there was sincere acceptance of the dual sovereignty theory, where the States sought to do their full part, and accepted the de- terminations of the National Government as to the rest, the plan worked. Where the States sought more from the Federal authority than it could give, and resisted national demands - then came dissension and, at length, war.