What event led to the implementation of the "3-1-1 rule"?

The implementation of the "3-1-1 rule" was a direct response to the actions of the individual commonly referred to as "The Underwear Bomber," who attempted to detonate explosives concealed in his underwear aboard a flight on December 25, 2009. This incident highlighted significant vulnerabilities in aviation security, prompting a reevaluation of security measures at airports.

The "3-1-1 rule" specifically refers to regulations concerning the transportation of liquids in carry-on luggage. Under this rule, travelers are allowed to bring liquids in containers of 3.4 ounces (100 milliliters) or less, all contained within a single, clear, quart-sized bag. This measure was enacted to minimize the risk of liquid explosives being smuggled onto airplanes, directly stemming from the tactics used in the Underwear Bomber's failed attack, which underscored the need for stricter limitations on the items passengers could carry onto flights.

The other events listed, while significant in the realm of security threats, did not directly lead to the urgency and specific implementation of the “3-1-1 rule” in the same way that the Underwear Bomber's actions did.