Why did Herbert Chapman pull his centre half back into defence?

Football's most famous tactical invention may have started with a rule change and a quiet idea from a veteran player. In June 1925 the offside law changed, and attackers suddenly needed fewer defenders between them and the goal. Arsenal's manager Herbert Chapman built a formation to answer it: the W-M, created in the mid-1920s to counter that change in the offside law. It is named after the letters the players' positions resemble on a diagram.

The key move was simple. Chapman's formation added a centre-back to stop the opposing centre-forward, who would otherwise have gained the most from the new law. The idea came from Charlie Buchan, a player at Chapman's Arsenal: move the centre half from a roaming spot in midfield to a stopper in defence. With the inside forwards brought back to help in midfield, the shape went from 2–3–5 to 3–4–3, a W and an M stacked on top of each other. The full backs were pushed wider, and the offside trap became the centre half's job.

Chapman was not the only one with the idea. Newcastle beat Arsenal 7–0 that season using a similar system. But Chapman refined it, joining it to counter-attacking football, pacy wingers and a strong defence. As he put it, the best time to score is immediately after repelling an attack, because opponents are then strung out in the wrong half of the field.

It took time. Arsenal spent most of the rest of the 1920s in mid-table while Chapman found players to fit the system. The five-year plan came to fruition on schedule: his Arsenal won their first major trophy in the 1930 FA Cup Final, and went on to win five League titles and two FA Cups in that decade.