Best strategy is as follows:
pork one of you enemies. make sure this doesn't require much of your player resources. just a couple of them are enough to increase the effort needed for the enemy to attack you to too high levels. this works especially well when the front line of enemy fiels is in your own zone. do NOT even try to capture his fields in this situation.
mass the rest of the players against the other country, focus on killing acks and vulching. if you get organized enough, maybe fhs. roll that enemy with numbers.
This works even with relatively low numbers. Porking one enemy will make them go away from your front and hit the other enemy. This will heavily weaken his defence, which makes rolling over him easier.
Gang bang is the best strategy there is, unfortunately.
edit: PS: most players don't seem to understand this strategy. as soon as the front fields of one enemy are porked, and the enemy has withdrawn, they start capturing his fields. this is bad for two reasons: the new front needs to be porked again and if you're moving him out of your zone effort needed to pork will increase 4-fold. also this will bring up some defence no matter how bad the fields are porked, which takes away from helping to gang bang the other enemy.
