During the Civil War
In 1861, Dahlgren's superior at the Navy Yard resigned to join the Confederate navy, and President Abraham Lincoln wanted to name commander Dahlgren to the post of Commander of the Washington Navy Yard. By law, however, that position could only be held by an officer with a rank of captain or above. Lincoln successfully persuaded Congress to pass a special act legalizing Dahlgren's appointment to the yard, and, in July 1862, Dahlgren was promoted to the rank of captain and made chief of the Bureau of Ordnance. Then in February 1863 Dahlgren was promoted to Rear Admiral and took command of the South Atlantic Blockading Squadron. In 1864, he helped William Tecumseh Sherman secure Savannah, Georgia.
Dahlgren's son Colonel Ulric Dahlgren was killed during the Civil War in a cavalry raid on Richmond, Virginia, while carrying out an alleged assassination plot against Jefferson Davis and the Confederate Cabinet. The plot is known as the Dahlgren Affair. The admiral was deeply troubled by his son's death and role in this event. Despite Radical Republican associations, John Dahlgren's younger brother Charles G. Dahlgren (1811–1888) was a strong proponent of slave ownership and was a Confederate Brigadier General, Commander of the 3rd Brigade, Army of Mississippi, which he personally funded.
After the Civil War
Dahlgren, took command of the South Pacific Squadron from Rear Admiral George F. Pearson, in 1867. When he was relieved of the command of the Squadron in 1869, he returned to the Washington Navy Yard where he served until his death in 1870.
Information from Wikipedia at: John_A._Dahlgren