Martial law was a system of rules that enables the military to take over the normal administration of justice. Martial law was declared in states of emergency, or at any time that events could lead to civil unrest within the population. Curfews were often put into effect during states of martial law.
Day 4 Edit
After Habib Marwan escaped from the Rockland Building, there was reason to suspect that he had another attack planned for the day. President Keeler insinuated to Secretary of Defense Heller that martial law might be an option to consider, due to the unprecedented level of civil unrest.
Day 5 Edit
A little after 6:00pm on Day 5, Vice President Hal Gardner requested that President Charles Logan enforce martial law in order to help aid the search for the nerve gas. Logan was at first skeptical about the idea, believing that martial law would encite chaos rather than contain it.
With added pressure from Gardner and the recent nerve gas attack on CTU, Logan declared martial law in Los Angeles at approximately 8:04pm. Later, Chloe O'Brian left CTU with help from Karen Hayes and Bill Buchanan and stayed at Bill's house for a short while. When Karen warned Bill that DHS agents were coming, Chloe escaped to a hotel bar while the curfew was still in effect.
The curfew remained in effect for the rest of the day, even though Logan believed the threat to be over after midnight.