Why destroy Laketown's bridge?

[Rikae]

Originally Posted by Nogrod

Or maybe the people were just thinking Smaug needed that other front to actually destroy the city? They might have been wrong but still acted as they acted according to their belief.

The trouble with that interpretation, though, is that Tolkien does tell us Smaug was foiled - so the Lakemen's action was effective in some way. Now, I suppose it's possible that they cut down the bridge to foil Smaug in one way while he was actually foiled in a completely different way (they thought he had one plan involving the bridge while he really had another), but the simplest and most likely explanation is that the plan they thought he had and the one he actually had were the same.