2.e4
Although unusual, the opening is fully sound. Perhaps the only reason it is not seen more often is that most players of the Nimzo-Indian, Queen's Gambit Declined, and Dutch prefer to not give White the option of playing 2.e4 after 1.d4, so they tend to play 1...Nf6, 1...d5, or 1...f5 instead. After 1.d4 e6 2.e4, Black must decide whether to play 2...d5 (the French Defence), 2...b6 (Owen's Defence), 2...c5 (the Franco-Benoni), or 2...f5 (the Kingston Defence).
Read more about this topic: Keres Defence