Bangladesh, July 7 -- On Saturday, July 4, Americans celebrated the nations 250th anniversary with the familiar fanfare: fireworks, flags, patriotic speeches, and ritual invocations of liberty. Yet at this extraordinary milestone, when the country is more deeply divided than at any time in recent memory over the meaning of democracy itself, it is worth asking what, precisely, we are celebrating.

The obvious answer is that we celebrate the birth of the United States. But that answer is incomplete. Every Fourth of July Americans commemorate not the Constitution but the Declaration of Independence. This is a remarkable fact, though we seldom pause to consider it. The Constitution governs us. Judges interpret it. Presidents swear to preserve...