Gregory P. Downs is professor of history at the University of California at Davis and the author of "After Appomattox: Military Occupation and the Ends of War," "Declarations of Dependence: The Long Reconstruction of Popular Politics in the South" and "Spit Baths."

In 1867, the nation’s capital was the site of a Second Revolution, one that breathed life into the promises of the first. (Saul Loeb/AFP/Getty Images)

Not many of the people who toasted the American Revolution on July 4 will gather Wednesday to celebrate the 150th anniversary of a key moment in the Second American Revolution: the long-forgotten Third Military Reconstruction Act passed on July 19, 1867.

It is not hard to see why people celebrate Independence Day and forget the period of Reconstruction after the Civil War, even though that period was, in many respects, a Second Founding that re-created the republic and the Constitution. Independence Day kindles thoughts of successful military struggle against a now-foreign enemy in service of famously high-minded ideas of life, liberty and the pursuit of happiness that make Americans proud.

The Second American Revolution, by contrast, pitted Americans against other Americans, Confederate slave owners, and came on the heels of a bloody conflict that ripped the nation asunder and still sparks conflict today. The Second Founders’ reliance on the military to police society and polling places, rather than to defeat enemies, also makes us queasy. And their foundational documents, such as the Third Military Reconstruction Act, read like enumerations of authority, not eloquent evocations of liberty.

Nevertheless, the events surrounding the Third Military Reconstruction Act may actually tell us as much, or even more, about this country, its potential and its predicaments, than the words penned in Philadelphia. For the act arose from a genuine constitutional crisis — a confrontation between a belligerent president and a cautious Congress over whether generals should follow the law or their increasingly unhinged commander in chief.

Their conflict turned upon still-enduring questions about whether the federal government could protect voting rights and create equality for former slaves and their descendants. Just as critically, the mundane mechanics of government embedded in the Third Military Reconstruction Act helped produce an extraordinary constitutional revolution in the 14th and then the 15th Amendments, transformations so powerful that the Senate pronounced them (along with the 13th Amendment) a “Second Founding” in a 2015 resolution.

In many ways, we live in a country shaped more by the Second Founders than by their better-known, but ultimately unsuccessful, predecessors in 1776. In Philadelphia, the first Founders created a nation that dissolved 85 years later. One hundred-fifty years ago, however, America’s Second Founders formed a Second American Republic that still survives.

As in any revolution (including the American Revolution), some parts of the past endured, but the violent, permanent remaking of the country introduced a new, persistent set of arguments over voting rights, citizenship, federal power and the unrealized potential for a multiracial republic.

The Third Military Reconstruction Act reminds us that revolutions advance through implementation, not just declarations. On its surface the bill is annoyingly mundane. Rather than sonorous phrases, it is full of stern reiterations of the “true intent and meaning” of prior legislation and formal approval of the Army’s ongoing interventions in the ex-Confederate states. No one quotes it. Few, even among historians, remember it. To see why it was so important, we have to look less at its language than at its context.

During the Civil War, ex-slaves, U.S. commanders and anti-slavery politicians destroyed slavery through blunt force and brave service. Yet Americans disagreed bitterly over exactly what would replace it and who would make those decisions.

After Confederate sympathizer John Wilkes Booth assassinated President Abraham Lincoln, the presidency passed to Vice President Andrew Johnson, a tempestuous and earthy Tennessee Unionist who had owned slaves and fought politically against planters. Johnson gave the former Confederate states leeway to construct a caste system in the South through Black Codes that excluded African Americans from testifying in court, voting, holding office or owning property in some locations.

Aghast, African American women and men across the country organized in Union Leagues and Republican clubs to articulate the key principles of their Second Founding, principles worked out in northern free black communities long before the war, especially the creation of legal and political equality. “Without the right of suffrage, we are without protection and liable to Combinations of outrage,” Washington, D.C., “Colored Citizens” wrote in December 1865.

That winter, Republican congressman Thaddeus Stevens tried to block the president’s plan. Stevens, a brilliant parliamentarian, sharp-tongued polemicist and longtime advocate for African American rights, allied with the more-staid Maine Sen. William Fessenden to refer all the ex-Confederate states to a special Joint Committee the two men headed. Together they kept the ex-Confederacy in a state of war and claimed congressional authority over their future.

Johnson responded wildly, suggesting to a crowd that Stevens should be hung as a traitor and comparing himself to Jesus Christ. After meeting famed ex-slave abolitionist Frederick Douglass, Johnson dismissed him as “just like any n—–.” When Johnson vetoed the Civil Rights Act and a bill extending the Freedmen’s Bureau to aid ex-slaves, Congress brusquely overrode him and ignored his wild claims that they discriminated against white people by guaranteeing rights to African Americans.

The standoff over the future of Reconstruction then went to the voters. In the 1866 midterm elections, Northerners backed the congressional Republicans, and they returned to Washington ready to take command. In March 1867, Stevens and Fessenden helped pass the first two Military Reconstruction Acts over Johnson’s vetoes.  These bills continued the state of war in the ex-Confederate states (except for Tennessee), placed them under military rule and ordered generals to register black men to vote in new constitutional conventions to remake state governments and ratify the 14th Amendment to the Constitution. Congress also banned Confederate officeholders from the polls.

But Johnson would not be cowed by legislation (or the will of the voters). His attorney general, Henry Stanbery, wantonly ignored Congress’ intent, declaring that generals lacked legal authority over state officials and that registration boards could not exclude Confederate officeholders. Johnson hoped to restrain the military and grind Reconstruction to a halt until court cases or the electorate could end it.

On the ground, soldiers faced a dilemma. Should they obey Congress’ law or the president’s orders? In Louisiana, Maj. Gen. Philip Sheridan followed Congress, dismissing the state governor and sweeping away discriminatory laws with the support of Secretary of War Edwin Stanton and commanding Gen. Ulysses S. Grant. But Grant feared that Johnson would undo this work.

To save Reconstruction, exasperated members of Congress returned to Washington for a special session. Their vehicle, the Third Military Reconstruction Act, looked prosaic precisely because of their concrete goals. They affirmed generals’ power over “so-called State governments,” and empowered registration boards to do their “duty” to exclude Confederate officials. In a bitter veto message, Johnson denounced the law as tyranny, the process as revolution. “It is impossible to conceive any state of society more intolerable than this,” he groused. Immediately the House and Senate overrode his veto.

For the moment, the revolution ground forward. Generals struck down discriminatory laws and removed foot-dragging state officials; registration boards enrolled freedmen. Ten former Confederate states called constitutional conventions that, for the first time, represented both black and white Southerners.

More than a quarter of the delegates to those conventions were black men, including former slaves such as war hero and future congressman Robert Smalls of Beaufort, S.C. These biracial conventions remade the South, creating the first public school systems in some states and outlawing whipping, imprisonment for debt and property qualifications for voting.

Then they remade the Constitution, ratifying the 14th Amendment that created birthright citizenship and guaranteed equal protection and due process, the cornerstone protections Americans enjoy today against state and local governments. Two years later, these ex-Confederate states provided crucial votes to ratify the 15th Amendment, protecting voting rights for the first time. “The history of the world affords no example of an equal success,” said Sen. Willard Warner of Alabama. “You cannot find an instance where so great a revolution has been wrought.”

Yet the Third Military Reconstruction Act was always a slender reed for such lofty hopes. When Johnson undermined the act by removing Sheridan and Stanton, Congress rushed to save what remained of Reconstruction. Wary of its ability to govern through the military in the face of Johnson’s intransigence, congressional Republicans recognized the new, biracial state governments as they ratified the 14th Amendment. It was not the complete overhaul that many had hoped for but seemed the best that could be obtained.

Then Stevens led the House effort to impeach the president. In the Senate, however, Fessenden and a few other moderates refused to go along, for fear of overturning the constitutional order and risking the republic’s survival. Johnson survived by a single vote, though in his last months in office Congress routinely and without discussion overrode his vetoes.

But the counterrevolution would be harder to stop on the ground in the South. Soon, a murderous insurgency claimed control of the region through assassination and coups. In 1888, a congressman estimated that 50,000 Southern African Americans had been killed over the past quarter-century.

Between the 1880s and early 1900s, Southern states hollowed out the constitutional guarantees of the Second American Revolution, passing laws that disenfranchised and segregated African Americans. Only in the 1950s and 1960s did the U.S. government finally make that Second Constitution meaningful.

If Reconstruction did not settle the question of citizenship, rights and equality, it introduced key issues Americans still argue about today: How can we protect voting rights when state governments try to restrict them? How can we shield military independence and provide congressional oversight against a lawless president? These are not new questions in U.S. history, and they are not insoluble ones.

Backed by the bland but crucial enforcement provisions of the Third Military Reconstruction Act, generals such as Grant and Sheridan defended congressionally enacted statute over presidential whim, politicians such as Thaddeus Stevens and even the cantankerous Fessenden remade themselves into revolutionaries by using military force to expand the Constitution, and Smalls and other black and white Republican community organizers created bold, new experiments in biracial democracy on the ground. Without the enforcement provisions of legislation such as the Third Military Reconstruction Act, the high ideals might have vanished into air. And the challenge of enforcing rights remains a crucial one in American life to this day.

Robert Smalls, the former slave turned congressman, lived long enough to see the limits of this revolution. Three decades after serving at the constitutional convention that the Military Reconstruction Acts called into being, he was one of a handful of African American delegates to the 1895 state convention that disenfranchised them. There, South Carolinians under “Pitchfork” Ben Tillman, a viciously racist governor and U.S. senator who praised lynching on the Senate floor, worked to undo the Second American Revolution. Futilely, Smalls urged them to “make a Constitution for all people, one we will be proud of and our children will receive with delight.”

Given the untidiness and disappointments of Reconstruction, and the historical amnesia around the role of slavery in the nation’s founding, it is no wonder that Americans celebrate the inadequate Constitution the First Founders wrote, and all too often overlook the Second Constitution that Smalls and so many others willed into being, and left in our hands to sustain and extend.  Both the lofty dreams of Smalls’s speech and the concrete mechanisms of the Third Military Reconstruction Act provide clues that may yet allow us to complete the revolution they commenced and create the country they hoped to will into being, a country capable not just of articulating rights but of defending them.