Alliances ... How to keep them?

[Quillan]

It's not guaranteed, and it does depend heavily upon your own faction choice, but high reputation goes a LONG way towards maintaining good diplomacy.

There's a thread buried here that lists all the modifiers, and I can't remember what the thread is titled, but here's some from memory:

Having a faction leader with high chivalry increases your reputation slightly each turn.
Having a faction leader with high dread lowers your reputation slightly each turn.
Anytime you take a city and occupy it, you gain reputation.
Anytime you exterminate a city you lose reputation.
Anytime you release prisoners (for whatever reason) you gain reputation.
Anytime you execute prisoners (no matter the reason) you lose reputation.
Every turn, you gain and lose reputation points for each faction in the game that has a relationship with you above neutral (so-so, I think is how it displays in game).
THE BIGGIE - anytime you break an alliance (no matter the reason), you lose MAJOR reputation.

So, make friends with everyone you can by giving gifts to them. Occupy every city, never sack or exterminate. Release all prisoners, never offer ransom because you'll take a hit if it's refused. Make one single alliance and stick to it. The things I'm saying are excessive, I know. However, with one caveat, this should help you maintain solid diplomacy throughout the game.

That one caveat is religion. If you have bad relations with a faction of another religion, it drags down your relations with ALL factions of that religion, and especially so if the religion is question is Catholicism. If the Pope doesn't like you, none of the Catholic states will like you. I've tried, but it's the next best thing to impossible to maintain high reputation playing as Byzantium. Venice won't like you, therefore the Pope won't like you, therefore the other Catholics won't like you, therefore your reputation will suffer.

The reason I say make only one alliance is that your reputation will be completely screwed if you have two allies that go to war with one another. You will not be able to maintain alliance with both, and breaking the one you chose to break will drop your reputation through the floor.

[Grog]

What Quillan above and others have said is all good advice.

To add a few points, based on my experiences running a max rep HRE game (immaculate when last timurid general died, on M/VH 1.2)

To maintain multiple allies longterm, you need to manage their goals as well as your own. Its fine to have allies bordering with you, as long as they don't become 'landlocked' by your empire ie: they always need to have somewhere to expand other than your lands.

I will tend to keep all my allies on 100/10 tribute+ occasional gifts, and renew it regularly. Small amount to pay when you consider the price of defensive wars, the thought of the tribute seems to count more than the actual amount gifted. You can also gift lands etc to keep paths open for your allies.

Another factor I have found useful is to buffer your allies against one another. Most of the alliances I have had broken is not my ally betraying my empire, but instead attacking a neighbouring empire I am allied with, forcing me to choose. To counter this, I will create territorial buffers ie: before I accept a further ally, I will buy/obtain lands that may be a problem with current allies, so they wont be at war with each other.

Finally, keep your allies busy. War by proxy works wonders for doing this. I have posted this technique before but its well worth repeating. ie: I am allied to france as HRE. France is at war with Milan. I wasnt really interested in Milan as I was making a pilgrim autobahn from HRE heartlands to the holy lands through diplomacy and defensive action. So, as soon as france seemed to be gaining on Milan, I would have diplomats fund Milan for a few turns to turn the tide. Once Milan gets strong, funding dries up and france gets more florins. Kept them busy for ~150 turns on and off with this tactic, plus its far cheaper than building/maintaing armies. To do all this you do need a good rep though...

[Rhyfelwyr]

I always get marriage alliances when I can, since the AI doesn't seem to break them so easily, in my Danish campaign I shared huge borders with Poland but they never attacked me despite having large armies nearby when I was rampaging across Britain.

It also seems to be easy to stay allied with the Pope, since the alliance and the Pope-o-meter seem to complement each other.

[CynicalP]

Is there a chart of the reputation levels and there relative effects of diplomacy? So far the highest I seen my reputation is trustworthy. I 'm usually hovering at highly Reliable throughout the campaign. I have been able to maintain most of the alliances throughout the camapaign. The only time I generally have problems is when two faction that I'm allied with go to war.

[Old Geezer]

The highest that I have had is "very trustworthy" and to get it I never attacked another catholic faction and allied with the Pope and every catholic faction that I could. When an allies went to war I always stayed with the one being attacked. Do not ever retake a city if you have given it or traded it to another faction, even if it is the target of a crusade or you will find as I did that your reputation can drop about 4 levels immediately and will not recover for hundreds of years, if ever. You can attack, loot, and exterminate factions with different religions with impunity, as long as you have not had relations with them.