1. Setting the Stage: September 11, 2001 was a traumatic day for the entire nation, but for Washington D.C., it was particularly traumatic since the city was targeted by one of the planes that crashed into the World Trade Center. American Airlines Flight 77 crashed into the Pentagon in Arlington, Virginia, just outside D.C, killing 184 people, including passengers and crew members as well as military personnel who were inside the Pentagon.

2. Rapid Response: Amidst the chaos, the city and its residents rallied, with first responders - police officers, firefighters, paramedics, and ambulance workers - rushing to the scene of the attack within minutes. In all, more than 25,000 people assisted in the response effort, including military forces, city officials, and good Samaritans.

3. Symbolic Site: The attack on the Pentagon was particularly significant since it was the headquarters of the U.S. Department of Defense, making it a high-profile target for terrorists. In fact, the section of the building that was hit was undergoing renovations aimed at strengthening the structure to withstand an attack.

4. Memorializing Lives Lost: The tragedy of September 11, 2001 continues to be memorialized in D.C. to this day, including the creation of the Pentagon Memorial, which features individual memorials to each of the 184 victims. The 9/11 Memorial Museum, located in New York City, also features an exhibition dedicated to the victims from the Pentagon attack.

5. Increased Security: In the aftermath of the September 11 attacks, the city of Washington D.C. ramped up its security measures to prevent future attacks from happening. This includes establishing a perimeter around the Pentagon and other high-profile government buildings, conducting more frequent security checks at airports, and spending hundreds of millions of dollars on security upgrades citywide.