Why were the lords of Westeros and Essos not more wary of the Targaryens? Dragons had acquired a... reputation for burning and destroying entire armies and cities.
Surely the lords of Westeros, especially those around the Blackwater, would have been... concerned over the possibility, no matter how small, of facing off against dragons.
Is there any comment on this by George R. R. Martin or any mention in his works?
The kings of the nearby Westerosi nations definitely had the raw manpower and resources required to overrun the Targaryens' meagre manpower, and it would have really been the wisest decision.
If the kings attacked Dragonstone when there was only Balerion, before Vhagar and Meraxes hatched, they could have stormed the keep and slaughtered everyone.
Brutal, yes, but the church likely would not have cared since the Targaryens worshipped the gods of Old Valyria at the time. The common folk also would not care because they were always too busy trying to store supplies for winter and serving their lords to care.