Here’s another way to use diplomacy in order to get what you want in this game:


Sometimes, when you’re feeling generous enough or are just role-playing, you just have a great desire to defend one of your allies.
Problem is, you don’t have a common land-border and therefore they don’t agree to exchange Military Access, so you can just watch helplessly from afar how the enemies lay siege to your ally’s capital, without the possibility of sending reinforcements.

Well, there’s a solution for that.

Create a common border, even if only temporary. For example, conquer Cordoba from the Moors and after that exchange military access with your allies Spain and Portugal (although the spaniards and the portuguese might declare war to each other later in the campaign, so it’s not very wise to ally both of them at the same time, but anyway, you get the point).

Even if you lose/sell/gift that province afterwards, the Military Access treaty still stands.
And you can march your armies through allies’ territory at will, getting’ them out of trouble every time it’s necessary.

You can also use this strategy in order to determine one of your allies to declare war to one of your enemies.
It is known that an AI faction will agree to attack another AI faction only if they have a common border, so you can conquer/buy a province that borders your enemies’ lands and then sell/gift it to that particular ally you want to drag in your war.

Might not work every single time, as your allies are sometimes reluctant to attack other factions, but it’s worth a try.