[1] He graduated from the United States Military Academy (West Point) in 1825, and received a commission as a second lieutenant in the 3rd Regiment of Artillery.

A few months after graduation, he became a private secretary to his older brother Richard Clough Anderson, Jr., who was serving as the US Minister to Gran Colombia.

He served in the Black Hawk War of 1832 as a colonel of Illinois volunteers, where he had the distinction of twice mustering Abraham Lincoln in and once out of army service.

[2] Returning to regular Army service as a first lieutenant in 1833, he served in the Second Seminole War as an assistant adjutant general on the staff of Winfield Scott, and was promoted to captain in October 1841.

He was severely wounded at Molino del Rey while assaulting enemy fortifications, for which he received a brevet promotion to major.

He then served from 1849 to 1851 as a member of the Board of Officers to devise "A Complete System of Instruction for Siege, Garrison, Seacoast, and Mountain Artillery," which was adopted on May 10, 1851.

[3] From 1855 to 1859, in view of his precarious health and probably also due to his connections to General Winfield Scott, Anderson was assigned to the light duty of inspecting the iron beams produced in a mill in Trenton, New Jersey for Federal construction projects.

Anderson took the fort's 33-star flag with him to New York City, where he participated in a Union Square patriotic rally that was the largest public gathering in North America until then.

The modern meaning of the American flag, according to Harold Holzer in 2007 and Adam Goodheart in 2011, was forged by Anderson's stand at Fort Sumter.

To punctuate this feast of national colors, New York's graphic artists rushed out patriotic engravings and lithographs depicting avenging soldiers or gowned goddesses, bayonets upthrust, carrying "The Flag of Our Union" into future battles that, at the time, could only be imagined.

Composers dedicated songs like "Our Country's Flag" to President Lincoln, and adorned their published sheet music with colorful images of resolute soldiers gripping the national banner.

The abstraction of the Union cause was transfigured into a physical thing: strips of cloth that millions of people would fight for, and many thousands die for.

Speed met with Anderson and found him reluctant to implement Lincoln's wishes to distribute rifles to Unionists in Kentucky.

Anderson's last military assignment was a brief period as commanding officer of Fort Adams in Newport, Rhode Island, in August 1863.

[19] Anderson was the great-grandfather of actor Montgomery Clift through his daughter Maria,[20] although this relationship has not been definitively established by genealogical sources.