Complex sentence: Archer was finally exchanged late in the summer of 1864, and rejoined the army, briefly commanding two brigades in the Siege of Petersburg before his health finally collapsed after the Battle of Peebles' Farm.
Split into simpler sentences: Archer was finally exchanged late in the summer of 1864, and rejoined the army. He traveled to Petersburg, Virginia, and command of his old brigade, briefly serving in the Siege of Petersburg before his health finally collapsed after the Battle of Peebles' Farm.