Sometimes, when playing Dawn of War II, if I do especially well, I pin the enemy in their own base. I generally play as a Techmarine, which gives me the ability to make turrets. There are few effective counters to such a thing early in the game (every army has an option available, but the cost can be quite staggering in some cases). When placed outside an enemy base, said army is faced with a few choices. They can either draw the fire with one (or more) unit(s) while attacking with others (though, considering that I can manually control the turret's targets, this is a fairly terrible option). They can use a stealth unit to move around behind (though I have the option of leaving a handful of guards to prevent this very thing). They can use an assault unit to bridge the gap (not every army has an assault unit at tier 1). They can just wait until they reach T2 and use any vehicle to overcome the problem (but this means their team mate will be forced to fight alone against significant odds, and in the best of the probable outcomes, they will have a severe resource disadvantage). Or they can wait for their team mate to assist (leaving at least one army free on the other side to gain a resource advantage).
In most cases, if I manage to push an army back into their base and am confident enough in my remaining strength to press deep into opposing territory, and withstand the inevitable counterattack with sufficient strength to hold my ground long enough to build the turret all without such a move being countered, chances were quite good the turret would have been overkill anyhow.
It isn't so much a dirty tactic as it is kicking someone when they're down. Nothing is more annoying in that game than being pinned in your base, except being pinned in your base while the opposition moves on to T2 well ahead of you, meaning even if you're freed you are now at a severe disadvantage.
