ROL = Rule of Law, WROL = Without Rule of Law, for some of our newer preppers.
No matter what the trigger event, if a global or nationwide catastrophe, the real problems we'll face are pretty much the same...
1. Grid going down (so includes all that entails)
2. Lack of resources due to trucks not rolling
3. Military law in urban centers
4. Desperate, hungry people that will do anything for resources
5. Lack of sanitation leading to disease and illness in cities
6. Eventually organized gangs looking to take resources from others (some of which are likely to be former law enforcement or military, now using their equipment to ensure their survival)
7. People who, without authority, will succumb to base instincts, including formerly unimaginable crimes.