An instructive moment from a recent game. What should white not play next?
(Edit: the editor won't let me make paragraph breaks today for some reason. Sorry about that!)
Why is it a mistake? We should go back to see what the minority attack is really for. Why does white pursue this plan in the Queen's Gambit. What is the goal of advancing the queenside pawns?
After all, the aim of the game is checkmate. How does the minority attack help white to achieve checkmate?
In the absence of a direct attack to the king, the best way to achieve checkmate is to get a decisive material superiority. The easiest way to do this is to take your opponent's pieces. However, it is not always possible to take your opponent's pieces because many chess players are on the lookout for having their pieces taken. So what else can you do?
Answer: if you advance a pawn to the opposite end of the board, it becomes a queen. Having an extra queen also gives you a decisive material superiority.
Idiotic as this sounds, it is actually a really useful guide to chess strategy. Most of the moves played by strong players which do not seem to be preparations for checkmate are aiming towards queening a pawn ... eventually. For example, we all know that you are supposed to put your rooks on open files. Why are you supposed to put your rooks on open files? Well, one idea is to move them to the enemy 7th rank and take the pawns that are there. These pawns will then no longer be in the way of your own pawns. Then you advance your own pawns and make some more queens. Then you win.
Obviously the minority attack is a very roundabout way of achieving checkmate. So it must be aimed at queening a pawn somehow. But wait, if you want to make a queen, shouldn't you be attacking on the wing where you have more pawns? Why would you want to attack on the side where you have a pawn minority, if you can't possibly make a new queen there?
This is a good question, and reveals why a minority attack isn't always a good idea. Sometimes exchanging pawns on the side where you have a minority just helps your opponent to get a passed pawn on that side. Then your opponent ends up with the extra queen and you lose.
But in the specific position of the Queen's Gambit, things are a bit different.
The idea is, starting with a pawn structure like the one in the first diagram below (couldn't work out how to get the kings off the board) white advances the b-pawn and exchanges it for black's c-pawn, resulting in the pawn structure in the second diagram. Then white either
- moves the rook the b7 and takes black's pawn
- takes the pawn on c6
- puts a piece on c5 where it hopefully exerts a powerful influence on the position
In either of the last two cases, black's pawn majority is immobilised and white can advance some pawns on the other side of the board, make a passed pawn, and get a new queen. Great!
So what is wrong with playing b5 in the first diagram in this post? It's no good because black can reply with c5 and has a fine position! It's true that black gets an isolated pawn, but white's pawn on b5 is far weaker. It's doing nothing and has advanced too far into black's position. In the game, black won material quite rapidly after ... c5.
(Besides, an isolated pawn isn't always a bad thing. One of the signs of strong players is not seeing doubled or isolated pawns as the bogeymen they aren't.)
So there you go; a lesson in grand strategy! And what eventually happened in the game, you ask? Black, pondering smugly on the long-term view of the game ... lost on time. Which illustrates another useful principle: tactics >> strategy. Always.