Taken as a prisoner of war on the first morning of the Battle of Gettysburg, Archer was the first general officer captured from Gen. Robert E. Lee's Army of Northern Virginia.

[1] Nicknamed "Sally" at Princeton for his frail and slight physique, he studied law at the University of Maryland and passed the bar exam.

In 1847, during the Mexican–American War, he was commissioned as a captain and served with the Regiment of Voltigeurs in numerous battles, being cited for bravery at Chapultepec and brevetted to the rank of major.

[2] Returning to Maryland, Archer resumed his law practice, but decided in 1855 to join the regular army as a captain in the 9th U.S. Infantry, with whom he served primarily in the Pacific Northwest.

His men made a forced march from Harpers Ferry and arrived in Sharpsburg on the left flank of the Union IX Corps.

Gen. William Dorsey Pender led an attack that drove a Union pursuit force back across the Potomac River, enabling Lee's army to slip away into Virginia.

[4] Archer, along with many other officers captured at Gettysburg, was eventually sent to the Johnson's Island prisoner of war camp on the coast of Lake Erie, where his health rapidly declined due to exposure to the inclement Ohio weather.

He wrote a letter to the Confederate War Department in which he advocated a plot to overthrow the guards, but the conspirators would require assistance from the government to get the men back home.

After a stay of nearly a year, he was sent, along with 600 officers from various prisons, to Fort Delaware, in accordance with a scheme to reship them to Morris Island in South Carolina, a place under constant fire from Confederate cannon.

[5] Instead, he resumed command of his old brigade, and briefly served in the Siege of Petersburg, until his health finally collapsed after the Battle of Peebles' Farm.