This September 1862 photo provided by the Library of Congress shows Allan Pinkerton on horseback during the Battle of Antietam, near Sharpsburg, Maryland. Before the outbreak of war, he had founded the Pinkerton National Detective Agency. In 1861, he famously foiled an alleged plot to assassinate president-elect Lincoln, and later served as the head of the Union Intelligence Service -- the forerunner of the U.S. Secret Service. #

Fort Sumter, South Carolina, April, 1861, under the Confederate flag. The first shots of the Civil War took place here, on April 12, 1861, as Confederate batteries opened fire on the Union fort, bombarding it for 34 straight hours. On April 13, Union forces surrendered and evacuated the fort. Union forces made many attempts to retake the fort throughout the war, but only took possession on February 22, 1865, after Confederate forces had evacuated Charleston. #

A view of Washington, District of Columbia, from the intersection of 3rd and Indiana Avenue, ca. 1863. In the foreground is Trinity Episcopal Church, in the background, the unfinished Capitol building. Construction on the capitol was briefly suspended early in the war, but continued through the later years. #

A March, 1863 photo of the USS Essex. The 1000-ton ironclad river gunboat, originally a steam-powered ferry, was acquired during the American Civil War by the US Army in 1861 for the Western Gunboat Flotilla. She was transferred to the US Navy in 1862 and participated in several operations on the Mississippi River, including the capture of Baton Rouge and Port Hudson in 1863. #

Inflation of the Intrepid, a hydrogen gas balloon used by the Union Army Balloon Corps for aerial reconnaissance. The the Balloon Corps operated a total of seven balloons, with the Intrepid being favored by Chief Aeronaut Thaddeus Lowe. #

The CSS Stonewall was a 1,390-ton ironclad built in Bordeaux, France, for the Confederate Navy in 1864. After she crossed the Atlantic, reaching Havana, Cuba, it was already May, 1865, and the war had ended. Spanish Authorities took possession, soon handing it over to the U.S. government. #

A view of Andersonville Prison, Georgia, on August 17, 1864. Andersonville was an infamous Confederate Prisoner-of-war camp, where nearly 13,000 of its approximately 45,000 Union prisoners died in brutal conditions, suffering from starvation, disease, and abuse from their captors. #

Dead horses surround the damaged Trostle House, results of the Battle of Gettysburg, in July of 1863. Union general Major General Daniel Sickles used the farmhouse as a headquarters and Union and Confederate troops fought among the farm buildings during the fierce battle. #

An execution in Washington, District of Columbia, on November 10, 1865. Henry Wirz, former commander of the Confederate prisoner of war camp near Andersonville, Georgia, was tried and hung after the war for conspiracy and murder related to his command of the notorious camp. #

Union forces of Benson's Battery in the Battle of Seven Pines stand guard in the fighting against Gen. Thomas "Stonewall" Jackson's Confederate troops at Fair Oaks, near Richmond, Virginia. The battle, also called Fair Oaks, took place on May 31 and June 1, 1862. #

Confederate dead lie among rifles and other gear, behind a stone wall at the foot of Marye's Heights near Fredericksburg, Virginia on May 3, 1863. Union forces penetrated the Confederate lines at this point, during the Second Battle of Fredericksburg. #

A black Union soldier sits, posted in front of a slave auction house on Whitehall Street in Atlanta, Georgia, in 1864. The sign reads "Auction & Negro Sales". View a closeup detail of this image here. #

In Atlanta, Georgia, soldiers sit atop boxcars at a railroad depot. At right is the office of Atlanta's Daily Intelligencer newspaper. Panorama made from two photographs taken by George N. Barnard in 1864. #

The deck and turret of the ironclad U.S.S. Monitor on the James River, Virginia, on July 9, 1862. the Monitor was the first ironclad warship commissioned by the U.S. Navy, and famously fought the Confederate ironclad CSS Virginia (built from the remnants of the USS Merrimack) in the Battle of Hampton Roads -- the first meeting in combat of ironclad warships -- on March 8-9, 1862. #

The camp of the Tennessee Colored Battery, pictured during the Siege of Vicksburg at Johnsonville, Tennessee, in 1864. View a closeup detail of this image here. #

Serving as a soldier in uniform and getting regular army pay, a former slave (center, with hands in pockets) stands with other Federal soldiers at the Army of the Potomac winter headquarters near Fredericksburg, Virginia, The log hut served as a mess house for the regiment. #

A view of the burned district of Richmond, Virginia, and the Capitol across the Canal Basin, in 1865. The city was assaulted by Union forces for more than nine months during the Siege of Petersburg, after which Confederate Gen. Robert E. Lee's army abandoned the battered city in April, 1865. #

The Baptist Church in Fredericksburg, Virginia, photographed from the backyard of the Sanitary Commission depot on May 20, 1864, after the city had been damaged in two different major battles of the war. #

The ruins of an extensively damaged Roundhouse in Atlanta, Georgia after the Atlanta Campaign in the summer of 1864. After Union Maj. Gen. William T. Sherman captured the city, he began his destructive March to the Sea, finally taking the port of Savannah on December 21. #

Residents walk through the ruins of Richmond, Virginia, in April of 1865. Richmond served as the capital of the Confederate States of America during the majority of the Civil War. After a long siege in 1865, with General Ulysses S. Grant's Union troops about to take the city, Confederate troops were ordered to evacuate, destroying bridges and burning supplies they they could not carry. A massive fire swept through Richmond, destroying large parts of the city. About one week after the evacuation of Richmond, Gen. Robert E. Lee surrendered to Grant in near Appomattox, Virginia, on April 9, 1865. #