Do you know that there are limits on the size of a kingdom that you can declare war on?
Utopia limit wars against smaller kingdoms to hinder those honorless slime who would gangbang kingdoms half their size or less. They also limit wars against larger kingdoms to protect the big guys against some small thief kingdoms who would declare war against the top 100 kingdoms just to be the ones who "took them down". Complaints used to fly in the main forums about how honorless it was to play a thief because of a handful of kingdoms like that.
Don't be afraid of losing a li'l land. You will recover from it quickly if you are exploring, and while your land is down your TPA is up permitting you to take back that lost fame, and then some.
Do not assassinate their wizards
unless they aggressively attack you with magic even then you will be amazed
how effective it is to send one mission assassinating them, then send them
a message warning them to back off or loose the bulk of their wizards the
next time. If you used diplomacy and they are still after you with excessive
spells, post their target data, and some record of the volume of their
spells against you. Odds are the other thieves in your kingdom will
be all too happy to help you slaughter their wizards in vast quantity.
Kidnapping or fireballs: When combined with storms this can be a devastating weapon against a province. Use it as a retaliation if necessary (again make use of diplomacy). After you walk off with 1/4-1/3 of their peasants now serving as your slaves, send them a message letting them know that it was simply a retaliation for grabbing your land, and that you have nothing against them. Remember that for every peasant of theirs you gain they have lost two. Losing 2/3 of your peasantry in under an hour will put the fear of thieves in you quickly. I have used it in combination with a polite message many times to deter significantly larger provinces from considering attacking me again. This is not however considered by many to be an honorable first strike. Athough it is worth a good bit of fame. So if you want to use it just to gain fame, spread it out over several targets. The loss of 1-200 peasants will phase few people unless they are already hurting but the loss of several thousands will hurt and anger any opponent who did not deserve such. Remember, if you are seriously trying to cause someone pain cast storms after abducting or frying their peasants.
Killing of soldiers? This is about as gray of an area as it gets. A few such missions will hardly be noticeable except that they will be reported as something to the effect of "XXX soldiers were found murdered in their beds". However, running this frequently against an opponent will cripple their ability to train new troops the following day. Some people realize this is no big deal, but there are others who believe anything more than stealing food, gold and runes to be "black ops", thus honorless and cowardly. As if there can be any honor in sending armies to take land from someone who does not have the defense to stop you even with all troops at home. *Sigh* What can we do with people who are so narrow-minded.
The stealing missions, incitement of riots, and the spying missions are generally accepted by everyone as part of typical game play. Except of course by those who feel anything a thief does is honorless.
Personally, I consider lightning strikes to be a waste... Those runes you spent casting the spell, and the runes of theirs that now can not be stolen would better serve the kingdom in other ways. Not everyone shares this opinion, and likes to steal all the runes they can from a wizard, then cast lightning strike more to take the mage out of the game a day or so.
Drought, vermin, storms these are generally fine to cast. Expose thieves can get your wizards slaughtered if you are not careful. Work closely with another spellcaster to take a thief out for the duration of the war.
Casting expose thieves a couple times so that you can break them is one thing, but I personally will have revenge against anyone that casts it excessively against me, and would suspect that anyone else would feel the same.
I think I've covered the "black ops" and "gray ops" sufficiently. Now this being said, make use of diplomacy. Play honorably and do what you can to gain as much as you can from war.
Remember the stealing missions gain you the most profit, but the grey and black ops gain you the most fame. Mix and match them to gain your desired effects.