The English Opening, and as response the Jaenisch Gambit. Chess.com has this to say: * It takes the c4 pawn off of a place where it attacks d5 It immediately allows one to fiancetto the c8 bishop if one is so inclined * the hole at b5 can be used sometime in the future to place the rook behind and attack with * It allows for an open, tactical game from the outset with its own unique character; the positions in which can be quite striking and fun to play * It will catch the English player off guard

Both advanced pawns under threat. The obvious defenses don't easily work; d3 bxc4 ; or Nc3 b5; or e5, and the black bishop gets even stronger. Maybe Qc2?

? The bishop has two jobs and can't do them both; g2 is weakened and the white queen's bishop blocked.

? Either Bc2 or Bxc4. This one just loses the e-pawn.

Necessary. Nf3 could've been better.

? Invites an exchange, or if declined, prepares d5.

White has edge in development, black hurries to catch up.

Gives black a tempo.

! The black queen has to protect c7. d6 is not a good square for it.

Invites Nxb4 Bxb4 Qe5 Nc6 Nf3, for an endgamish situation except for the black advantage in pawns.