how i can join to defender on Reconquista?
If you're militarily powerful: use "extort tribute" to tributise the defender, then join the war.
If all else fails, you could try getting an alliance with the defender. (Marriage/betrothal of close relatives gives non-aggression pact; then use the right-click menu to form an alliance.)
Caveat: I haven't played in Iberia since HF, so the above is entirely theoretical.
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