Why was move 26 in Carlsen-Anand 2014 WC Game 6 a blunder by both players? In the Anand vs Carlsen Match 6, why was move 26 a blunder for both players?

[White "Carlsen"]
[Black "Anand"]
[FEN "6rr/1k3p2/1pb1p1np/p1p1P2R/2P3R1/2P1B3/P1B2PP1/2K5 w - - 0 1"]

1.Kd2? a4?
Carlsen's 26.Kd2 is a a blunder because it exposes the White King to a tactic that wins two pawns and gives Black terrific pressure. The tactic is effectively a guarantee of victory for Black:

[FEN "6rr/1k3p2/1pb1p1np/p1p1P2R/2P3R1/2P1B3/P1BK1PP1/8 b - - 0 1"]

1...Nxe5! 2.Rxg8 (2.Rxe5? Rxg4) Nxc4+! {The Knight gets out of danger, with check and an extra pawn--you couldn't ask for anything better!} 3.Kd3 (3.Ke2 Rxg8) Nb2+ 4.Ke2 Rxg8 {Black is now two full pawns ahead, with lots of pressure on the weak g2 pawn and vastly better pawns for the endgame. }


The point is that 2...Nxc4+ wins Black a second free pawn, by virtue of the check against the White King. Without the ability to get the Knight out of danger with check, ...Nxe5 is not possible (because White could then simply trade Rooks and then recapture the Knight with a winning position).



Anand's mistake on his twenty-sixth move was that he missed this tactic, playing 26...a4 very quickly and allowing Carlsen to correct his mistake by moving his King away from d2.