[Werthead]

As others have said. Attacking the North gave the ironborn easy pickings, access to hostages and supplies and put them in the good graces of whoever won in the South once the time came to work out the post-war settlements. I'm pondering if Balon even did it to set up a future bargain where he gave the North back to the victor in the war in return for the Iron Islands' continue independence.

As for Euron's actions, again they make sense but only from a more limited point-of-view. Devastating the Tyrell homelands forces the Tyrells to send their fleet back to the west coast and creates dissension in the Tyrell armies, thus leading to the war not being properly concluded (the Tyrells failed to retake Storm's End, for example, and the Riverlands are still not fully pacified), thus dividing the mainlanders and giving the ironborn more time to act (and more time for their emissaries to get to Meereen and capture Dany's dragons). Actually, things have gone even better than expected, with Cersei and Margaery's arrest by the Faith giving the Tyrells a third situation to deal with.

I'm also wondering if the ironbron objective was to capture the Arbor and take the Redwyne family hostage? If so, then the Redwyne fleet's ability to intervene may be limited (as Paxter can be held to ransom for his family's safety).