Defending the 2 knights attack by attacking
Black often does Bf5 in the Main line.
Black can do other moves, but Bf5 is the normal way of playing it.
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The 2 Knight Attack tries to counter the Bf5 line.
It sets a trap of sorts against it.
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So the lines are completely different
What line was you wanting to talk about exactly?
Bf5 Main line or 2 Knight Attack line
I'm really not a fan of the main line for white (maybe calling it the old main line is more accurate these days). There aren't alot of options for white other than the theory, it also doesn't branch very much... so black can just learn the theory and get a decent position they'll be very familiar with.
Not really a huge fan of the two knights attack for white either, since if black just plays the opening precisely you can wind up in a position where whites momentum has fizzled out. It's also a pretty common line these days.
The only line I really like after an early Nc3 is the rasa-studier gambit. But generally I think an early Nc3 is quite bad against the caro-kann specifically. Unless it's 1. Nc3 and you can play a Jobava.