Pentagon History: 7 Big Things to Know > US DEPARTMENT OF DEFENSE > Story – Department of Defense

The Pentagon is a universal symbol of U.S. strength and security known around the world, but do you know the history behind it?

The 1,100 acres of land on which the Pentagon sits was once part of the sprawling estate of Confederate Gen. Robert E. Lee. The federal government confiscated it during the Civil War, but the building's concept and construction didn't happen until several decades later.

Here's the rest, in a nutshell.

The Pentagon was the brainchild of Army Brig. Gen. Brehon B. Sommervell, who, in the early 1940s, pitched it as a temporary solution to the then-War Department's critical shortage of space as the threat of joining World War II became imminent.

The plan was approved, and on Sept. 11, 1941, construction began. About 296 acres of land were designated for the building, which was supposed to be turned into a hospital, office or warehouse once World War II was over.

The logic behind this was pretty simple the site designated for construction was bordered by five roadways, so the developers decided to go with a five-sided building. Naturally, they called it the Pentagon.

The grounds and building went up in a stunning 16 months. The building was officially completed in January 1943, thanks to the help of 1,000 architects and 14,000 tradesmen who worked three shifts around the clock. A staggering amount of materials were needed, too, including:

The first tenants moved into the building in April 1942, several months before the building was finished.

At the height of World War II, the Pentagon housed more than 33,000 people, and its worth exceeded expectations. Officials discovered that they did, in fact, need to keep such a large military force active once the war was over. So, instead of turning the building into something else, it remained the military's command center.

Nowadays, it houses the Office of the Secretary of Defense, the Joint Chiefs of Staff and the highest echelons of the Army, Navy, Marine Corps and Air Force.

In 1992, the building was designated a National Historic Landmark. Due to its age, renovations began around that time on the building's 4 million square feet of space.

On Sept. 11, 2001, the nation's sense of security was shattered when terrorists hijacked commercial airplanes and flew them into New York City's World Trade Center, into a field in rural Pennsylvania, and into the nation's symbol of military might.

American Airlines Flight 77 crashed into the west side of the Pentagon, penetrating three of its five rings and killing 184 people. Thankfully, several offices in that area weren't occupied because of the ongoing renovations. In fact, one of the sections had just finished getting upgrades that improved security features, including walls and windows with greater blast resistance. An initial analysis after the attacks suggested that helped to save a lot of lives.

The team working those initial renovations was tasked with rebuilding the damaged portions. The reconstruction effort was dubbed the Phoenix Project, and it cost $500 million.

By Aug. 15, 2002 less than a year later the first tenants whose offices were damaged began to return, even though renovations were nowhere near complete.

Aside from the rebuild, a groundbreaking for a memorial honoring the lives of the 184 killed at the Pentagon on 9/11 began June 15, 2006. The memorial, which was dedicated on Sept. 11, 2008, sits on a two-acre plot of land on the southwest side of the building, next to where the airplane struck that fateful day.

Even before the terrorist attacks, the renovations were considered a massive undertaking, which included:

According to Pentagon historians, 40,000 personnel had to be relocated during the project's various phases, which went as follows: Each wedge was broken into 10,000 square feet; plumbers, electricians and other tradesmen then had five days to complete their tasks before moving onto the next 10,000 square-foot section.

The first section was completed in 26 weeks to accommodate a timetable laid out by Congress.

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Pentagon History: 7 Big Things to Know > US DEPARTMENT OF DEFENSE > Story - Department of Defense