Principle 1: The point of fighting a war is to destroy your opponent as quickly as possible in order to stop the war. War hurts your NW and your goal should be to end it quickly.
Principle 2: Walk Softly and carry a big stick. Try for peace first. Let your threats be subtle. Let them know you would rather not fight but will do what you have to defend yourself.
Principle 3: If you are not prepared, you will not win. This is where having adequate defense, at least 10% guilds, and a good thief ratio really pays off. Friends and allies you've made all game should be brought in. Good communications between the kingdom should of been established at this point.
Principle 4: Spread the Pain. Every participating member of the other kingdom (if not all members of the other kingdom) must be damaged. Until a province is hurt they have no real incentive to stop the war.
A) However, if a province does not attack and vows to stay out of the fighting, do not attack them. Every province that does not attack you gives you a numbers advantage over the enemy.
Principle 5: Know your enemy and yourself. Gather all the information you can, post it to your forum and then use it to your advantage. If you find an opponent with great defense and thieves but weak wizards, tear him up with magic. If he has weak thieves and is strong in defnense and magic, hit him with thieves. Find the weak spot your opponent has and hit them in that weak spot. If you find an opponent who is strong in all area's let your strongest thief and mage specialists hit them. Make sure your people with great thief and mage ratios are hitting the people on the other side with good ratios that most of the people on your kingdom can't get through on.
Principle 6: Bring your allies in. As I mentioned earlier, avoid war as much as possible. But if it happens, bring in everyone you can. Trust me, I have never fought in a war where the other side didn't bring in help and/or multis. You've got to do the same thing to give yourself the best possible chance of victory.
Raze Attacks: If there's a province on the other side that's a little too large for you to hit or if you don't want to take on anymore land, raze attacks are a nice option. This would be especially good if there's a large province you haven't been able to get through on yet who you could damage with a raze attack.
Learn Attacks: These are good to further break down an opponents defense while gaining valuable sciences as well. This is a good attack to use when your opponent has already been quite large and has been beaten down because they may have a large amount of sciences available.
Arson - Use the most over powered attack in the game to your advantage. Burn your opponents homes to the ground at every opportunity. Focus your attacks on the largest provinces on the other side, preferably the ones that you can't penetrate with military attacks. If your opponent is a Fairie, I'd suggest that you hit them with a mystic vortex or two if possible to keep clear sight from stopping you. Keep in mind that your losses and failure rate go up drastically after you drop below 76 to 80% stealth or so. So unless you have a huge advantage over your opponent in your ratio you may want to tread lightly here. Also don't waste your thieves trying to arson targets you can't get through on. Spreading the arson around also makes enemy morale plummet like a rock.
Survey, Infiltrate, and Spy military - This is an important piece of intelligence gathering that should be used on large and medium sized provinces. Keep in mind that you only need to send 100 thieves or so to get a fairly accurate count on these missions. So they are great to run after your mana is down and you don't want to risk losing a lot of thieves. This information should be posted in your forum.
Assassinate Wizards - This is mainly a minor annoyance unless a large concentrated effort by several provinces is made to kill an opponents wizards. If you do kill enough that leaves your opponent wide open for spells.
Rob the Towers, Rob Granaries, and Rob Vaults - Any of these could be used against a province but are of limited use because other provinces can simply send them more food, cash, etc.
Night Strike - Not worth bothering with unless your opponent has a huge amount of untrained soliders for some reason (1k plus at least). If they have enough soldiers this can be a damaging attack for your opponent.
Incite Riots - Reduces their income a little bit. Mainly an annoyance attack. If you want to spread the attacks around and have low stealth this could be of use in hitting several targets just to show they can be hurt without risk of losing a lot of thieves. It will do limited damage but may be useful psychologically.
Sink Boats - This is less effective than most people think against a prepared opponent. In my experience people go half way with this and end up making their opponents just small enough so that they have to pound the newbies and small fry because they don't have the boats to hit the large targets. My advice is that if you're going to do this, do it hard. Have several provinces go after their boats and drive them down to nothing. Doing this halfway can actually hurt your kingdom's small and mid-sized provinces.
Kidnappings - Along with fireballs and storm this is a great follow-up to arson. Once a lot of homes are burned the peasants will drop at 3% per hour unless you take em down faster. You can quickly dry up an opponents cashflow and peasants to draft from with kidnapping.
Protective Spells - It's a given that you should cast these on yourself. Protection can be just enough to beat back an attack. Clear sight can foil an arson attempt and intimidate your opponent into not trying again. Nature's Blessing keeps the food rolling in and stops those annoying storms. Magic shield is very useful in lowering your opponents success % against you. There's no need to go into a lot of detail, you should know to cast these to protect yourself.
Crystal Ball and Crystal Eye - You need to CB and CE your adversaries constantly. This is essential to your success. CE has another use. If you look at your adversaries paper and see other kingdoms they've attacked or that have attacked them, write them and tell them you are at war. Invite them to join with you in destroying the kingdom. You may be able to get some help like that.
Nightmares - Of limited use unless your opponent has a lot of untrained soliders.
Vermin and Drought - This is great if you find an opponent who is either away or very vulnerable. By vulnerable I would say that if you have an opponent with little food, not enough farms (check with survey and do the math), and who has perhaps just been hit by a raze attack (to destroy farms). I would also suggest casting these spells at some point right after they have been online or when they are going to be away for a while (cast at midnight for example and hope he's gone to bed) . This will allow the lack of food to do maximum damage to your opponents military. Most people are very lax about checking their food as well. If you mix these spells in with several others, they may not even bother to look. I will tell you that it is difficult to do massive damage by starving an opponent because a lot of provinces have spare food and if he asks for it he'll surely get enough to feed his province unless he's way, way short on farms.
Lightning - Unless your opponent has a ridiculous amount of runes this spell is not very useful. You may spend more casting it that your opponent loses.
Expose Thieves - This is a great spell. It lowers your adversaries stealth. If you have a province that is doing a lot of thief damage you and some others in your kingdom can literally drive their stealth down in the teens (I've seen it happen). This is also good for lowering a provinces defense against thieves by lowering their stealth. However, this spell is best used in bunches. 2 or 3 times may help but it's not enough to stop an opponent cold or really soften them up for thief attacks.10-12 times is taking a thief province out of the game for a while or leaving someone wide to be thieved.
Amnesia - I once got a province near the end of a round for 100k points of science with Amnesia spells. This spell really irritates the Hell out of people. It takes points directly off of their NW and trust me, they are calculating every GC they spent on that science. This is a spell that really breaks the will to fight of a lot of provinces.
Fireball - A great offensive spell that is a terrific follow-up to arson. If you find someone who has a high population strategy this is a particularly satisfying spell to cast. Watching 2500 peasants turn to charred cinders with a single fireball takes a real bite out of an opponents income and draft. This is a good spell to spread around the damage with as well.
Storms - This is another good spell to follow up arson, kidnappings, or fireballs with. It keeps your opponents population from growing back quickly. So let's say you kill 10k peasants with fireballs and kidnappings. If you hit them with storms as a follow up, the peasants will take forever to return at a mere 1% per Utopian day.
Mystic Vortex - A good spell to use to soften your opponents up. This is particularly useful vs. Faeries to get clear sight out of your thieves way.