Why were lords in Westeros not more wary of the Targaryen dragons pre-conquest?

Question

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.

Answer 1

They may have heard of Dragons, but had never actually seen them in action. Dragons, as a species, don't seem to be native to Westeros. And in all recorded history, only the Valyrians were successful in training them to be weapons of war. During their long reign, the Valyrian Freeholds seem to have had little contact with Westeros. Their only incursion there being their small colony in Dragon Stone. Also, a whole century had passed between the Doom of Valyria and the Targaryen conquest of Westeros. That's a hundred years with no known record of dragons being used in warfare.

So the Westerosi kings had no first hand (or even second hand) experience with dragons at all, which is why they were so unprepared for Aegon I and his beasts.

Answer 2

I've been thinking about this and have come to the conclusion that there was nothing they could have done to prevent what Aegon The Conqueror did.

Since they got rudely evicted from Valyria by Sauron (who needed Mt Doom); The Targaryens had been predominately living in Dragonstone. This was in approximately 140BC (roughly 440 years ago). This gives the Westerosi kings and inhabitants at least 140 years to see dragons and their dragon riders roaming around their lands and getting up their usual Targaryen mischief.

You'd be stupid or blind not to notice the slowly encroaching dynasty and their pets; but what were they to do?

When the War of Conquest came:

• Harren The Black had built Orthanc Harrenhal
  • A supposedly impenetrable castle
  • Got burned to a crisp by dragonfire
• The Arryns lived in The Eyrie
  • Another supposedly impenetrable fortress
  • Easily taken over by dragons from above
• True to form, the Lannisters decided to take on the dragons head-to-head
  • An army of 55,000 men lead by King Loren I Lannister
  • 4,000 men burned to a crisp
  • Event known as "The Field of Fire"
• King Torrhen Stark tried to take Aegon on as well
  • After seeing Balerion The Black Dread, he did a nope and decided to bend the knee instead.
• The Dornish
  • Ahh, what a fight they put up, in the end no burnings or conquerings, but they decided they want to be friends and married into each other

From start to finish, Aegon's conquest took a maximum of 3 years, this is ridiculously fast given that the events of the War of The Five Kings has been going for 3 years and counting!

In conclusion
We cannot say whether the Westerosi were concerned about the Targaryens and their dragons, however, the point is moot, since there wasn't much they could have done to prevent what happened.