Grammatically, the phrase functions as a proper noun phrase. The date, "11 sep 2001," acts as an adjectival modifier specifying the noun "pentagon." Together, they denote a singular, significant historical event: the terrorist attack on the headquarters of the United States Department of Defense in Arlington, Virginia. On that morning, American Airlines Flight 77, a Boeing 757, was hijacked by five al-Qaeda terrorists and deliberately crashed into the building's western side.
The impact occurred at 9:37 a.m. EDT, penetrating three of the building's five concentric rings. The crash and the subsequent jet-fuel-fed fire caused a partial collapse of the damaged section. The attack resulted in 189 fatalities: all 64 people aboard the aircraft (including the 5 hijackers) and 125 military and civilian personnel inside the building. This event was one of four coordinated attacks carried out against the United States on that day, alongside the two aircraft that struck the World Trade Center towers and a fourth that crashed in a field in Shanksville, Pennsylvania.
The strike on a primary symbol of U.S. military power was a pivotal moment in American history. It was a direct catalyst for the launch of the "War on Terror" and the subsequent U.S. invasion of Afghanistan. The physical damage to the structure was extensive, but an immediate and rapid reconstruction effort, known as "Project Phoenix," saw the damaged sections rebuilt and reoccupied by the first anniversary of the attack. A permanent outdoor memorial was later constructed on the site to honor the victims.
