The letter that 47 Senate Republicans addressed to Iranian leaders this week, warning them about making a nuclear deal with President Obama, came as a surprise to many Americans. But it would not have surprised our earliest forefathers. After all, it was not uncommon, in the years immediately following the American Revolution, for individual Americans to negotiate directly with representatives of foreign governments.

Many then doubted that the United States would hold onto its western settlements and remain a single country. In 1786, Congressman James White of North Carolina told the Spanish diplomatic envoy Diego de Gardoqui that if the United States made a treaty with Spain that did not guarantee Americans access to the Mississippi River and the port of New Orleans, far western North Carolina might declare independence and swear an oath of allegiance to Spain. In the spring of 1788, a group from Tennessee and Kentucky, seeking an alliance with the Creek Indians, also declared themselves willing to break away from the United States and become Spanish subjects.

Other Americans independently committed forces to foreign wars. In 1793, the French emissary, Edmond-Charles Genet, recruited citizens in South Carolina to raise forces to fight with the French against Britain and Spain. In 1797, Senator William Blount of Tennessee plotted to invade Spanish Florida with help from the British.

But these attempted negotiations taught Americans and foreigners alike that a decentralized system of foreign relations did not work.

The potential secessionists could never work out an agreement they liked with Catholic Spain. And the national government could not prevent or control war. When Georgians moved west onto Creek Indian land in the mid-1780s and fighting broke out, the War Department could only lamely ask Georgia to let it mediate. But instead of accepting federal authority, the governor of Georgia called out the state militia. Georgia found itself in the middle of a war it could not win alone. ...