Modern Attack (3.d4)

Petrov's Defense

1.e4 e5 2.Nf3 Nf6 3.d4 Nxe4 4.Bd3 d5 5.Nxe5 Nd7 6.Nxd7 Bxd7 7.O-O

White ignores e5 and strikes in the center with 3.d4; Black takes on e4 and the game opens up.

Fun fact: Wilhelm Steinitz recommended 3.d4 in The Modern Chess Instructor (1889). Source

The idea

White opens the center with 3.d4. The main line is 3...Nxe4 4.Bd3 d5 5.Nxe5 Nd7 6.Nxd7 Bxd7 7.O-O: both knights have traded, and White is a little ahead in development.

Wikipedia calls 3.d4 the Steinitz Variation; Lichess calls it the Modern Attack.

Plans for White

Use the lead in development: Re1, c4 and Nc3 put pressure on the e-file and on d5. The bishop on d3 points at Black's kingside, so a later attack is possible.

Plans for Black

Finish development with ...Bd6 or ...Be7 and castle. Black's knight on e4 is strong but can be challenged, so be ready to support it or trade it on good terms.

Watch out for

For Black, the e-file. With White's rook coming to e1 and Black's knight on e4, pins and discovered attacks on the e-file are the main danger before Black castles.

For White, the d4 pawn. After the exchanges, it can become isolated and a long-term target if White doesn't keep the pieces active.