Imagine if some well organised terrorists mounted an attack on the power system - targeting power generation, transformers and transmission lines.
You don't have to imagine that - just look at the city where I live - Christchurch, New Zealand.
4 September, 4:35am, a 7.1 mag earthquake hit us. That is the same magnitude as the Haiti quake.
80% of people from the mountains to the sea lost power. Many got it back by nightfall, most by the end of the weekend, some had no power for weeks. Farmers were particularly hard off, since they couldn't milk cows.
Most of the city's water was contaminated from broken sewerage pipes, so we had to boil our tap water (those that had tap water!) for almost a week. Lots of people were getting water from tankers and using portaloos two and three weeks after the quake.
And, to make it even more stressful, the aftershocks are still occurring (and will do so for months afterward). These are delaying rebuilding - you can't fix a building if an aftershock is going to wreck it halfway through the repairs.
As a tramper (what us Kiwis call hikers) I had lots of dehy food, gas bottles, camping stove, water purifaction tablets, water filter - the works. Ironic, since I was in one of the few streets that only lost power for a few minutes.
Without electricity, modern life is extremely difficult. No banking or ATMs or EFTPOS, so buying stuff is hard. The batteries on many cell towers ran out, so cell phone coverage was spotty. No traffic lights, combined with damaged roads, so driving was difficult (the authorities made it pretty clear - unless you really have to drive, don't!). Even now there are streets with restricted speed limits because the roads are damaged. Rubberneckers turned out to be a major hazard - blocking emergency traffic, damaging roads further and just plan -ing off residents by gawking at damaged homes and lives.
In all, it was a pretty small taste of SHTF, but an important wakeup call for lots and lots of Cantabrians.
Building on my previous point in this thread, what was very helpful was the community. Neighbours coming over and asking how you were. Offering my gas stoves to my neighbours. Lots and lots of people leaving their damaged homes and heading to community centres to help out others. People baking muffins and taking them around all the police manning the checkpoints. A bunch of university students organising via Facebook to buy shovels and go around helping clear silt. I believe community is what will get us through SHTF situations.
B