EDITED:
8. Stormlands have lost their last stronghold 100 days ago and their faction hasn't been officially eliminated, yet. Something's wrong here.
9. It's day 154 (Static Wars) and Dragonstone has not declared war yet upon the Westerlands, nor the Westerlands haven't made an alliance with the Reach.
The letter you bring to Stannis should trigger the war between Dragonstone and the Westerlands.
8. Stormlands have lost their last stronghold 100 days ago and their faction hasn't been officially eliminated, yet. Something's wrong here.
9. It's day 154 (Static Wars) and Dragonstone has not declared war yet upon the Westerlands, nor the Westerlands haven't made an alliance with the Reach.
The letter you bring to Stannis should trigger the war between Dragonstone and the Westerlands.