How does guarantor's strength's evaluation work?

[RedArmy]

It is 1475, I play Granada, I conquered Tlemcen, and got 4 vassals, Toggourt (which I fed Mzab's provinces), Fezzan, Djerid, and Tafilalet. Tunis and Ottos are my allies. I have just completely crushed Aragon (I took them Valencia and two other provinces and a lot of money), and also Morocco (I took them Fez and two other provinces and a lot of money).

I want now to guarantee Aragon and Morocco so that Portugal and Castille either don't attack or attack them and drag me into a defensive war. However, I can't do that because the tooltip says I am not strong enough... I don't understand because both of these nations have heavy debts, rebels, no navy, a very small army like 3k soldiers, and basically 0 manpower. I can pay for 3 advisors, I have a 15k army, and a 16 boats navy... I don't understand!

 

[Vin55]

It is has something to do with economic situation e.g income and Force Limit not standing army. So that can be the reason why.
But I would not guarante them. Just wait and see if Portugal attacks them and if they do send an alliance or just savescum. But the guarantee sets a new peace timer so not really worth it. Especially when it is land you whant need.

 

[RedArmy]

My force limit is indeed quite low... And I own only 5% of Seville trade node.

If you want to know my story’s end, I decided to attack Souss and Tunis because they were allied to them. So I lost Tunis as an ally. Portugal attacked Morocco and Castille attacked Aragon. I couldn’t do anything to protect them. But then, Castille stupidly decided to attack me alone while Ottos were at war with the mamluks! They nonetheless backed me, I just waited the Ottos’ army to show up in Iberia and roll over Castille, this was fun to look at! I took a few provinces, money, and broke their alliance with France. Then I decided to attack Portugal with my vassals only, Castille tries to defend them but they have zero manpower. I think I will restore Al-Andalus in a few years!