The Alekhine Defense is a chess opening which belongs to the half open games.
It begins with the moves:
Black does not develop his e-pawn (1...e5 or 1...e6) but attacks the white center pawn instead to provoke him to move ahead attacking and chasing the knight, expecting to create weaknesses in White's position as the e-pawn will be attacked and traded off later on.
This idea originates from chess world champion Alexander Alekhine who used it 1921 in a tournament in Budapest. This chess opening is quite bizarre and ignores chess principles of the old school as Black allows the opponent to chase his knight around as he pleases right after the first move. White builds up an impressing pawn formation in the center which will be attacked later by Black. The balance and symmetry of the position is completely disturbed and only good players will be able to cope with the complexities of the position.
The Alekhine Defense has a respectable reputation and was played by Robert Fischer, Victor Korchnoi, Vassily Ivanchuk, Shabalov, Minasian, Lev Alburt, Aronian, Adams and Nakamura. At the moment it is not very popular but this is just a matter of fashion.
Main variations are the Chase-Variation, Modern Variation, Exchange Variation and the Four Pawns Attack.
The next moves are usually:
Chase-Variation (Lasker Attack): 3.c4 Nb6 4.c5
The Modern Variation: 3.d4 d6 4.Nf3
The Exchange Variation: 3.d4 d6 4.c4 Nb6 5.exd6 gives White some space advantage. Black plays ...g6 followed by ...Bg7 and ...Bg4
The Four Pawns Attack: 3.d4 d6 4.c4 Nb6 5.f4 gives White even more space. Black can play ...Qd7 followed ...0-0-0 and ...f6 pressures the pawn formation. Or Black plays ...Nb4 followed by ...c5
White tries to secure his space advantage and Black tries to undermine and destroy the pawn formation using his pieces. Black must play active or his position will deteriorate due to the white center control.
A further disadvantage is that the kingside of Black is not properly defended anymore, as the king-knight has moved to the queenside. This can lead to insufficient protection of the black king.