The Marshall Trap is the most common trap that you will see in the Petrov Defense. For those familiar with the Petrov Defense you know that it is a very drawish and somewhat passive opening. With that said, there is still room for error on both sides and the Marshall Trap focuses in on a common error that white makes in his development.
After white starts the main line by capturing our e5 pawn, the moves that follow are pretty straight forward and you will see them more than any other move order. This is why this is the most common trap in the Petrov. The trap comes when white is forced to choose how to attack the black knight on e4. The best move is to develop his own knight to c3 but it is also tempting to develop the rook to e1. Anytime you can bring your rook to an open file it is usually recommended but in this case it is actually a huge mistake. White is able to sacrifice his bishop and in the end gain a substantial material advantage due to a small error in white’s development that fell right into the trap.
Watch the video below to see a detailed explanation of the Marshall Trap.