This Guide is based upon my experiences within BF2 and is my advice and opinion.
Before you read this please repeat after me: Being on Defense is not a Bad thing. If you agree with that statement then please continue reading.
In Battlefield 2 the role of Defense is often highly underrated. For many the role of defense is often the wicked step child in online war games. In this I disagree. There are many different reasons why players do not want to defend captured flags. Some clutch onto their CTF experience and believe that defending is Flag Camping and so look down upon it. Others believe that defending is simply too boring and hence never do it. Others enjoy the offensive capacity much more than the defending ability and hence don't do it. And still others want more points and hence don't defend because the points are less forthcoming. Regardless of the reason people don't defend the main point is still that many don't defend.
Defense is a major role in strategy within the real world military as well as in online war games. A good offensive is useless without at least a moderate defense. Though the flow of the game is quite a bit faster than the real world and hence tactics are modified due to the pace. I hope to include certain strategies that I feel should be practiced in order to achieve a winning and solid game. In the following I am assuming there is a Squad structure and that this squad structure is followed. However even in a weakened state of structure as long as even a small group are following these methods there should be a noticeable increase in wins for that team.
The common practice should always be to defend the areas where the most likely counterattack will take place or hold the most important strategic value. Obviously bases that are not able to be captured do not play into this but truthful should hold at least a small defense team to prevent Spec Ops from destroying Commander assets or to stop the random spawn raper.
The ideal strategy on the battlefield would be to have an offensive team of one or two squads who would attack a location while a third squad would defend the closest captured flag or the last captured flag they took. These two locations would be the most logical location of a counter attack. The benefits of defending these locations is to provide your units with a closer base of operations. Hence if the offensive attack fails then they can launch another offensive from a close location therefore keeping the pressure on the enemy that is needed to keep them at their location. Forcing your enemy into a hurried counter attack with damaged vehicles or men or routing them is better than letting them orchestrate a planned assault. In this strategy the defense of the last captured flag is a very important one. It will usually ensure your victory on the next enemy location. A good way to ensure that everyone gets action would be to have one squad take a flag and capture it. They would then hold it until the second squad comes in and reinforces them. Then after a bit of healing and resupply the second squad would attack the next flag while the first squad remains in defense. Two or three squads moving in this fashion, alternating the offensive role, would not only give everyone points but also ensure that the recently taken flag is defended against a counter attack.
An important note to state is that not all locations should be defended. Only locations that are most likely to experience a counter attack or hold an important strategic value should be defended. These location include those closest to enemy positions or bases that hold key vehicles such as aircraft or a decent assortment of armor and vehicles. As there are no real choke hold points based upon geography in BF2 I can't seriously comment on base importance dependent upon geographical locations. However I can say that on certain maps elevation is very important in the role of spotting enemy. Giving your team forewarning of incoming enemy is always useful.
As I stated not all locations should be defended. A watchful commander who is doing their job will notice troops attempting to move around the map and take locations that are not defended. There should always be a mobile squad that is in place for just this circumstance. When the enemy is noticed moving to a undefended location the commander would spot them and then order a squad to defend or move to that location. It might only be for a few minutes to take out two guys in a jeep but its still a necessary tactic on the battlefield. Another obvious possibility would be to spawn at the location of that base and defend it. You can then make your way to the front-lines again when it is secure.
After many hours in the role of defense on many maps and in many flag locations I have come up with a few basic strategies that should help in the more secure defense of a flag location. Each location has it's own flair and key defensive locations. Not all are able to be defended effectively with just one squad but some can easily be defended with only a handful of players. First note where all the possible entry points are. This includes fences that have breakaway portions for infantry entrance, large openings where vehicles can pass through and roads leading to the flag location. Next determine where all vehicles spawn, where each MG, TOW or SAM location is. These are important to note so that you can quickly man them if need be. Lastly of course know where the flag is and how to get to it.
Defenders should not be huddled together in a single mass. This goes for being outside, on top of buildings or within buildings. This will make your squad more susceptible to ambush, grenade and artillery attack. The most protected and hidden member of the squad should always be the Squad Leader. Everyone else is expendable and can remain in higher risk locations. In the event that the base is taken the squad can then spawn on the Squad Leader and launch an immediate counter attack. This will often surprise the enemy and result in your squad recapturing the flag.
All coordinate directions from which the enemy can approach should be watched from an elevated position if possible or a hidden ground position. There should exist a good spread of Anti-Infantry and Anti-Vehicle roles as both will always appear. The Anti-Tank class is preferred but an Engineer or Spec Ops is a good secondary. If you are defending without some kind of Anti-Vehicle class and there is not a good TOW installation then your defense will probably fail. If good TOW and Machine gun posts exist then someone should be close by them. A strategically placed FAV/HUMVEE/APC can also fill the roll of a stationary defense platform.
Those on the perimeter should keep an eye out and Manually Spot ALL enemy infantry and vehicles immediately before dealing with them. This can be achieved by placing your aiming retinal on the enemy unit/vehicle, pressing the Q key and clicking Spotting. This way your team defending can react accordingly. There is no excuse for a vehicle to come rolling in with only one or two people on the defending team knowing about it. Spot the enemy FIRST and then move to act against them. This is practice that must be used for an effective and proactive defense.
Do not be afraid to change classes. If you feel that situation calls for more Anti-Tank then if you happen to die re-spawn as an Anti-Tank. If the situation calls for Assault or something else then re-spawn with that class. Do not forget that you can change classes as needed. This strategy has often broken the back of a solid enemy offense.