An extensive personal account of his military training and his participation in the Civil War was rediscovered long after his death and published in 1989 as Fighting for the Confederacy.

Alexander, known to his friends as Porter, was born in Washington, Georgia, into a wealthy and distinguished family of planters from the Antebellum South.

[3] He graduated from the United States Military Academy at West Point in 1857,[4] third in his class of 38 cadets, and was brevetted a second lieutenant of engineers.

[7] Lt. Alexander's final assignments for the U.S. Army were at Fort Steilacoom, in the Washington Territory,[8] and at Alcatraz Island, near San Francisco, California.

[3] At the First Battle of Bull Run, Alexander made history by being the first to use signal flags to transmit a message during combat over a long distance.

[10] Upon receiving a similar message, Beauregard and Gen. Joseph E. Johnston sent timely reinforcements that turned the tide of battle in the Confederates' favor.

He was also active in signal work and intelligence gathering, dealing extensively with spies operating around Washington, D.C.[9][11] During the early days of the Peninsula Campaign of 1862, Alexander continued as chief of ordnance under Johnston, but he also fought at the Battle of Williamsburg, under Maj. Gen. James Longstreet.

[3] He was instrumental in arranging the artillery in defense of Marye's Heights at the Battle of Fredericksburg in December 1862, which proved to be the decisive factor in the Confederate victory.

He personally arrived too late to participate in the battle but served as Longstreet's chief of artillery in the subsequent Knoxville Campaign and in the Department of East Tennessee in early 1864.

He served in all the battles of the Overland Campaign, and when Lt. Gen. Ulysses S. Grant slipped around Lee's army to cross the James River and assault Petersburg, Alexander was able to move his guns quickly through the lines, emplacing them to repel the main attack.

[9] During the Siege of Petersburg, Alexander had to adapt his artillery tactics to trench warfare, including experimentation with various types of mortars.

After unsuccessful attempts were made to locate the tunneling activity, the Battle of the Crater caught the Confederates by surprise although it ended in a significant Union defeat.

Alexander was a member of the boards on the navigation of the Columbia River, Oregon, and on the ship canal between Chesapeake and Delaware bays, from 1892 to 1894.

In May 1897, President Cleveland appointed Alexander as the arbiter of the commission tasked with fixing and demarcating the boundary between the Republics of Nicaragua and Costa Rica, with a view towards the possible construction of an interoceanic canal to be dug across Central America.

Alexander spent two years at the head of that commission, headquartered in the coastal village of Greytown (now San Juan de Nicaragua).

[20] Alexander was selected to give the Confederate veteran's speech on Alumni Day during the centennial celebration at the United States Military Academy on June 9, 1902.

He wrote many magazine articles and published his Military Memoirs of a Confederate: A Critical Narrative (1907), praised by Douglas Southall Freeman as "altogether the best critique of the operations of the Army of Northern Virginia.

Unlike such Confederate officers as Jubal Early and William Pendleton, Alexander eschewed the bitter Lost Cause theories of why the South was doomed to fail, given the overwhelming superiority of the North.

David Eicher called Fighting for the Confederacy "a superb personal narrative with a good deal of analysis of Lee's operations ...