Oh, something interesting I found. Best way to wear someone down, if you don't particularly care about driving them out, is to send a group just barely over a fifth of their combat strength. They'll take more casualties than you in most cases. If you have an extra personality, especially an Aggresive one, it just gets better.
Example: No one commands 1000 troops, and are sent to attacks a player province at Control 3, with 2000 troops in it. Effective combat strength is 1000, while the defender's is 4000. A little over a fifth, just to be on the safe side. They lose, of course, and take, let's say 19% Casualties, or 190 people. Their opponent takes, let's say, 7% casualties. Or about 140 people. And these are pretty favorable conditions for the defender, the only thing making it better for him being them using 3999 troops, or IS being more favorable towards the Defender and less for the Aggressor. And while there's a discrepancy in the number of men lost between the two, it's not much, and a war of attrition will favor the aggressor if he has higher income. If. But this is also between two players.
But now let's look at a more current view, and a more favorable one for the aggressor. Also, one that doesn't involve us trying to kill each other. Only Ghazkull. Let's say I had an Aggressive personality taking command of troops and attacking Drawno, which now has 12,400 troops in it, So I would only need 830 troops to reach over 1/5th combat strength. My effective combat strength would be 2490, and theirs would be 12400. One fifth of 12400 is 2480. Outnumbered over ten to one, they are handily defeated, but with grave losses. The Rebels take 23% casualties, if we go off my IS, or about 191 people die. On the other hand, the Rostovian's take 15% casualties. Or about 1860 people. Ten to one ratio. Those rebels sold their lives dearly, let me tell you. And of course, these are pretty favorable conditions for the attacker, the only possible improvement being if they had 10 IS, rather than the crappy 6 IS I have.
Next turn, that player/I could repeat the process, recruiting 66 people and sending 705(we/they had more than 800 people there in the first place after all) people this time. Possibly with another player spreading Propaganda or Investing in the province to improve IS for them/me. This time, then, 2115 combat strength goes up against 10540. Still over a fifth. Rebels lose, of course, but harass the Rostovian's and take, let's say the IS improved by 2(either two people used it and changed it by one, or a Writer spread Propaganda) so 19% casualties. 134 funerals. The Rostovians, on the other hand, truly have reason to mourn, for they just lost 15% of their force again, or 1581 soldiers. They've only got 8959 men there now.
Next turn, it repeats, and again, let's just say the IS improved by two. It's now at full, so we recruit 29 people and invade again, with 600 troops. 1800 combat strength against 8959, lose again, both sides take 15% casualties. 90 Rebels die, and 1344 Rostovians aren't going home to their families. Next turn, they don't even have to recruit anyone. 510 troops is enough to reach over one fifth strength. But now the Rostovians are getting smart. They've claimed what they needed to elsewhere in the last three turns, and just want to wait for reinforcements to increase. So they Husband Resources. Strength 7615 defeats strength 1530. 7.5% casualties this time, 15% for the Witkovians, and 77 Freedom Fighters become Martyrs, whilst 572 Husbands, Fathers, and Sons become sources of tragedy for their families.
They aren't being forced out of the province. But they're still taking incredible casualties. Is this a particularly morale tactic? No. It's the equivalent of suicide bombing. But now you see why suicide bombing is so effective.
tl;dr: Send barely enough troops to actually get a fight and you kill more of them than they kill of you. Or, to put it another way, strap explosives to people and send them running in with shotguns and grenade launchers.