Back in 1962, Frank Smith Jr. left Morehouse College in Atlanta and went to Mississippi as a civil rights activist with the Student Nonviolent Coordinating Committee. In Holly Springs, he met a man who turned out to be the descendant of an African American Civil War veteran.

“I found it ironic that a guy whose grandfather had fought to end slavery and preserve the Union was still being treated like a second-class citizen, not even allowed to vote,” Smith told me recently. “So I started reading about the soldiers.”

A seed was planted that would become the African American Civil War Museum and Memorial, which Smith founded in Washington in 1998. On Monday, nearly 50 years after that chance encounter in Mississippi, he’ll preside over the dedication of the museum’s expansion into a renovated school building at 1925 Vermont Ave. NW. It is the largest museum of its kind in the country and the only national memorial to black soldiers who fought in the Civil War.

“The Civil War ought to be one of the things that black people celebrate,” Smith said. “But we tend to think of Confederate flags instead of thinking about those 209,145 black people who fought for freedom and to preserve the union, 23 winning the Congressional Medal of Honor and coming out with three important amendments to the Constitution — the 13th, 14th and 15th — which ended slavery, gave blacks equal protection under the law and black men the right to vote. It was phenomenal.”

Smith, 68, is a native of Newnan, Ga. He moved to the District in January 1968, when he took a job as a researcher with the Institute for Policy Studies. That April, Martin Luther King Jr. was assassinated, and riots erupted in the city.

“I started to move back to Mississippi,” Smith said. “These people were burning down their own neighborhoods. I recognized that there was a lot of anger and bitterness, but I don’t think those black Civil War soldiers made the ultimate sacrifices so we could destroy our own homes.”

In 1982, Smith, a Democrat, was elected to the D.C. Council on a platform of providing housing subsidies and tax incentives for businesses. He represented Ward 1, where the museum is located, until 1998. During that time, construction on the Metro subway system’s Green Line began. Many businesses were hurt when streets were torn up to make way for a tunnel.

“I began complaining to Metro: ‘You have to build something special to make up for the damage you have done,’ ” Smith recalled. He and his constituents agreed on a black Civil War memorial with open space for community gatherings, which Metro helped fund.

The museum was a mere 700 square feet. The new one is more than seven times that size, with 5,000 square feet.

A sculpture by Ed Hamilton honoring the soldiers and sailors was added, along with a “Wall of Honor” that includes the names of the black men who fought in the war. More than 36,000 died.

The museum’s reopening coincides with the nation’s commemoration of the 150th anniversary of the Civil War. Also, on July 18, 1863, the Union’s all-black 54th Massachusetts Volunteer Infantry staged its legendary assault on the Confederate battery at Fort Wagner in South Carolina.

That was the group featured in the 1989 movie “Glory,” starring Denzel Washington and Morgan Freeman, with Matthew Broderick portraying Col. Robert Gould Shaw, who led them into battle.

The museum is in the Shaw neighborhood, named for the colonel. It began as a freed-slave encampment in the 1800s and became a black cultural mecca before the riots.

“After people visit the museum and the monument, they walk out into the community and see Howard University, Vermont Avenue Baptist Church, the Masonic Temple — all historic places made possible because of the guys whose names are listed on that memorial,” Smith said.

The museum features military documents, letters from soldiers, uniforms, rifles, photographs and tools for tracing family history. Smith and the museum’s staff recently launched a four-year campaign to find and mark the gravesites of the veterans.

“America would have been a much different country if the Confederates had won the Civil War — a divided and weaker place,” Smith said. “I don’t think the nation realizes how much black soldiers did to help make America great. Our job is to get the story out.”