In order to become involved in the struggle you need to fulfill ALL of the following conditions:
- have your capital in Iberia (that's the peninsula made up mostly of Spain and Portugal today, including the Balearic islands)
- have one of the involved faiths
- have one of the involved cultures
Since the achievement requires you to start uninvolved you need to start from outside Ibera. The only condition here is that your realm capital is not on the peninsula. So in order to get there fast you should be able to start in 867 as Duke Antso III 'the Terrible' of Gascogne in the South-West of West Francia. He is Catholic (involved faith) and Basque (involved culture). I haven't tried it that way so please let me know if that works.
It's easiest to start as a huge empire from Europe or Africa though.
I started as Chima Labidiedo of Fika (just south of Kanem) in 867, going for
in the same game, and built up the Empire of Kanem-Bornu containing all of Africa before expanding into Iberia.
To become involved you need to meet all the requirements above.
- have your capital in Iberia
That's the easiest part: Have a county in Iberia that is in your personal domain, don't be at war and have not moved your capital in your current lifetime. Now select your Iberian county and click on the "Move Realm Capital here" icon next to the name of the
castle, not the county.
- have one of the involved faiths
You need to have or adopt one of the involved faiths. You can get a list of the involved faiths (and cultures) in the struggle menu. Unfortunately creating a new faith while only being an interloper does not work.
I went with Mozarabism in order to also go for
Make sure your reign is safe, you're not going to die anytime soon and you are well liked before you take this step, otherwise your realm could fall apart from revolts.
- have one of the involved cultures
If you didn't start with an involved culture you can either take the "Convert to Local Culture" decision after moving your realm capital or - better yet - create a hybrid culture from your own and one of the involved Iberian cultures. Your current culture cannot be younger than 50 years since you last formed a hybrid culture of diverged your culture.
Now coming from a powerful empire the conquest way of ending the conflict is the most obvious. Basically just conquer all of the Iberian peninsula and the islands. You will also need to meet the following additional requirements while doing so:
- Hold and completely control at least 2 De Jure Kingdoms of Hispania
Just make sure not to give all the kingdom titles away, you need to personally hold them for this to count. Alternatively you can
- Unite the Spanish Thrones under your primary title
but that didn't trigger after I had all of Iberia conquered, probably need to create the title Empire of Hispania.
- All counties in the kingdom in which you have your realm capital need to be converted to your faith and culture.
It's best to have your realm capital in a rather small kingdom to make this go much faster. Then convert faith and culture using your respective councilors.
You should already fulfill the other requirements if you went for conquest.
Now you just need to wait for the struggle to move to a Hostility phase. Conquering counties, usurping titles, ransoming involved rulers and building stuff in any castle in your realm (and some other actions) gains you progress towards Hostility.
You can check your progress in the Iberian Struggle menu "View Ending Decisions". When you meet all the requirements the decision will be prompted and after you take it and resume the game your achievement will pop, along with