Not even attacks on infrastructure. Cities have alarmingly little amount of food, most cities only have a few DAYS of food in grocery stores. Get a natural disaster, a decent strike, thread of a hurricane (notice how shelves get bare on the news in cities expecting hurricanes... it's not because everyone is coming and buying 10 cases of water each and 100 cans of beans and 40 gallons of milk, it's because everyone is coming and buying a few days of groceries at once. Go to groceries at random times, figure out when their trucks come, you'll be able to go 2-3x a week and see bare shelves around the time the trucks are scheduled to come).