"It appears therefore that the several Hellenic communities, comprising not only those who first acquired the name, city by city, as they came to understand each other, but also those who assumed it afterwards as the name of the whole people, were before the Trojan war prevented by their want of strength and the absence of mutual intercourse from displaying any collective action.”
I'm not sure what this means.
Thucydides loved his dependent clauses...
What he is saying is that, before the Trojan War, the cities that were Hellenic in culture/nature were still too isolated from each other to have a common 'thrust' to their culture, or to act in concert on important matters. After the Trojans were defeated, cultural interchange could happen more freely and the really common Hellenic identity took hold throughout the cities in question.
He's saying that the Greek city states before the Trojan War operated only as individual polities and had never allied in any military actions collectively. Troy was the first time they all fought together as "Greeks."