Sorry, this post became an accidental strategy guide book. But as someone who finds the game quite easy on Very Hard I thought I’d share some of my decision-making process, and how thinking about it that way makes the game easy. Hopefully someone finds it helpful!
Once you realize how the AI makes its decisions in terms of who to War-Declare, you will actually have a much easier time on the higher campaign difficulties. Because a 3 front war is suboptimal until such a point as you have about 6 stacks and can just expand in every direction at once. By which point you’ve won.
Geographic distance/opportunity. The AI looks for where it is close to your weak points. This is pretty obvious but worth saying anyways. It theoretically theory crafts its chances for an attack on a city to work before it issues move orders. Even something basic like walling a minor city near potential hostiles before moving your army out of the general area will save you needless war-decs, because of these calculations.
Lord/Faction traits and bias. All the factions have AI traits that functionally make the AI more amenable to peaceful treaties with your faction or less. It’s helpful for your decision making to know which directions can you safely expand in and which borders will always be volatile. For example, you always want to get rid of factions like Wood Elves at the highest priority because they have some kind of specific Wood Elf AI behavior that makes them go from loving you to warring you in the matter of one turn.
Your “Trustworthiness” level. This is a HUGE wild-card factor. If you break any treaties, or declare a war outside of the 7 turns window of ending a NAP, the AI is going to hate you in general and make many more war-decs than it normally would. All the AI will, not just the one you betrayed.
If you consider all those factors before deciding where to plan your expansion, you can actually come to a pretty logical decision even with factions that seem to begin the game surrounded geographically. The Vampire Counts come to mind here. If you defeat the neighboring vampires and conquer their territory, you can virtually always get the dwarves to your east and south to not only trade treaty you (turns out they hate vampires in general so much that vampires who kill other vampires are ok in their books, I guess), but NAP you.
You have one city (Waldenhof I believe) that if you wall in the northern Sylvania, you will basically never hear from the dwarves again (*edited to add that the reason for this is AI generally won’t attack major cities like Drakenhof/Altdorf etc without massive armies, they consider them unwinnable fights, and Drakenhof in the South and Waldenhof are the two cities the AI is checking to attack from the dwarves). The closest human city to the north is from a weak faction that won’t be interested in you for a bit, and Waldenhof is also the point those AI’s will check for weakness. So with Walling one city, killing some other vampires, and trading with some dwarves, you have secured the North, East, and South. Well. The game just got super easy.
Plan ahead, especially with factions like Vampire Counts who start with some NAPs, so you can cancel them 7 turns prior to your planned invasion with the least possible disruption to the hidden diplomacy number of Trustworthiness. You’ll find most factions have a plan like this where they can crush in one obvious direction leaving a minimal defense at home, so you can really snowball by taking 1-2 cities per turn quite early in the game.