A while back I wrote a post on Terrain Analysis. If you happen to be a maneuver officer assigned to a vehicular based organization, the first O in OAKOC would be "Obstacles".
Obstacles are "neutral" on the battlefield. Anything that makes it hard for the enemy to get in makes it hard for you to get out. Obstacles come in two flavors, existing and reinforcing. Existing obstacles are anything that was there when you got there, and this can include buildings, bridges, canals, roadways, or muddy plowed fields. Reinforcing obstacles are what either you or the enemy put there after someone decided to have a fight. Make no mistake, putting up obstacles means you are fighting, even if the other side might not know it just yet.
Obstacles are key to an "aggressive defense" and can be a huge combat power multiplier.
Minefields are the very definition of "dumb bombs" and there is a very good moral argument to removing them from the arsenals of world powers. But on a smaller scale, things like tank ditches, nine strand concertina, barb wire fences, vehicle caltrops, abatis, and spike strips all have utility for a defender seeking to delay an attacker or canalize movement.
To get smart on obstacles, you don't need to know what the obstacle IS, but what the obstacle is INTENDED TO DO. Let me say that in a different way, because this is important. Obstacles exist to get the enemy to do some sort of action. The two most common effects is to SLOW MOVEMENT or to DETOUR MOVEMENT into a course of action desirable to the ones who laid the obstacle.
Think about that, a tank ditch won't STOP a tank, but it will slow it down significantly while the tank digs itself out by going back and forth to collapse the sides of the ditch into a traversable slope. If you are going to SLOW the enemy somewhere, WHY are you doing that, WHAT TACTICAL advantage does slowing the enemy at a certain time or place give you?
Lets think about it, either you have slowed the enemies advance out of a kill zone, or you have given yourself more time to unass your position before it is over run.
If you want to control where the enemy is moving, maybe it is so that you can channel him into a kill zone.
A while back I wrote about the War Fighting Functions. Obstacles are one way to negate an enemy advantage in Movement and Maneuver, or enhance your own advantage in M2. You figure out what you WANT to achieve, then you consult your handy engineering manual and try to figure out if you have time and resources to create an obstacle that will make the enemy do what you want. If you are lucky enough to have a Sapper handy with an FM 5-34 he can look up the data right quick and give you work/time/resources estimate.
If you don't have a Sapper handy, and have limited resources, then you need to get smart in a hurry on your terrain. Because even though it feels like there are a gajillion IEDs in Afghanistan, the truth is that they enemy only places them where they know ISAF will roll. It takes a lot of precious resources to make bombs, and so they are emplaced with plenty of forethought into the effect desired.
Lastly, obstacles that work on dismounted troops such as a tanglefoot, would do nothing against a tank. A tank ditch does nothing to slow dismounted infantry. You need to know what is coming at you in order to plan a good defense with integrated obstacles. That open muddy field won't stop either tanks or infantry, but a standard issue Detroit SUV might get stuck pretty quick. Maybe that's why so many Police Agencies are being fielded M113s?