During my recent trip to the big city of Chicago I noticed a one thing.
This entire country is a soft target. I saw hundreds of water towers that would be really easy to blow up with a rocket launcher. Or they could be easily attacked in the middle of the night. It probably would not take much to poke a hole in one of those. It would take very little to knock out a water tower and remove water for several thousands of people. Many of them were easily seen from the interstate highway, which would probably be the way the terrorists would arrive and escape.
It would take very little to cause very large amounts of destruction and slow down traffic in certain areas. Remember when that bridge was knocked down on I-40 by a barge. Very few people were killed but it slowed traffic and costed the state and federal government millions of dollars to fix the bridge. It would just take a really big hole in the road to make it useless to traffic or slow traffic down.
As a society we would be unprepared to fight a major ground based war or even fight a small war against a determined enemy using guerilla tactics in our cities. The very freedom we have creates huge gaping holes to attack us in our cities and homes.