Many of the impacts of a terrorist event last long after the event itself has finished.
Loss of Life & Property Damage
This is the main impact of a terrorist event. But as we know, casualties are not the only way terrorists can achieve their goals.
Long Term Psychological Damage
This can be accomplished with just threats and no real action on the part of terrorists. Although harder to quantify, the psychological aspect can be a huge impact to a nation.
Loss of Business, Either Due to Real or Perceived Threats
When people stop leaving their homes and carrying out their lives as normal due to real or perceived terrorist threats, businesses see the impact on their bottom line.
Allocation of Limited Government Resources to Deal With Real or Perceived Terrorist Threats
When the government sends resources to deal with domestic threats, that removes those resources from another area. Personnel such as police, firefighters, EMTs, and search & rescue teams are specialized professionals who are a limited resource.