Final answer:
The single event on U.S. soil that re-instilled fear in nations across a mostly peaceful world was the September 11, 2001 terrorist attacks carried out by Al Qaeda.
Step-by-step explanation:
The single event on U.S. soil that re-instilled fear in nations across a mostly peaceful world was the terrorist attacks that occurred on September 11, 2001. These attacks, carried out by the terrorist group Al Qaeda, targeted the World Trade Center in New York City and the Pentagon in Washington, D.C. The attacks resulted in the deaths of nearly 3,000 people and caused widespread shock and fear globally.
The 9/11 attacks had a profound impact on global politics and security. They led to the U.S. launching the War on Terror, with military interventions in Afghanistan and later Iraq. The attacks also prompted nations around the world to enhance their security measures and cooperate more closely in counterterrorism efforts.
The fear instilled by 9/11 was due to the demonstration of the vulnerability of even the most powerful nations to terrorist attacks and the realization that no country was completely immune to such acts of violence.