It's genuinely difficult now to explain how strange Arrigo Sacchi's Milan looked at the time. The ideas that side introduced are so completely absorbed into normal football that describing them sounds like describing water. Compact lines. High defensive line. Pressing. Zonal marking. Every under-13s coach in the world teaches this stuff.

In Italy in 1987 it was close to heresy. And the reason is worth understanding, because the orthodoxy it replaced was not stupid — it was a coherent, successful system that had won a great deal.

What he was arguing against

The prevailing Italian model was built around man-marking with a free sweeper behind. Each defender was responsible for an opponent, followed him wherever he went, and behind them all the libero cleaned up whatever got through.

It worked. It had won World Cups and European Cups. Its logic was defensive certainty: every attacker is accounted for, and there's insurance behind. The cost was that the defence's shape was dictated by the opposition's movement, and that the team was structurally reactive — you couldn't press, because pressing requires collective movement and your defenders were all following individual people in different directions.

It also produced a particular kind of match. Cautious, low-scoring, decided by moments. Whether that's good football is a matter of taste, but it was undeniably effective and deeply embedded.

The four reference points

Sacchi's counter-proposal was that defenders should orient themselves to four things, in order: the ball, the space, the opponent, and their teammates. Not the opponent first. The ball first.

That reordering is the whole revolution, and it sounds abstract until you see what it produces. If every defender is oriented to the ball rather than to a man, then the whole line moves together, because the ball is a single object and everyone can see it. Move the ball left, the entire team shifts left, maintaining distances. Play it backwards, the entire team steps up.

Once you have that, pressing becomes possible for the first time. You cannot press with a man-marking system — there's no collective, only eleven individual duels. With a ball-oriented system the team can advance as a block, and a block advancing at the right moment is what a press is.

The twenty-five metres

The most quoted principle from that Milan side is the distance between the defensive line and the forward line: about 25 metres, maintained at all times.

The reasoning is elegant. If the whole team occupies a 25-metre band, then wherever the ball is within that band, there are teammates within a few seconds of it. Pressing works because support is immediate. And critically, the space between the lines — the pocket where creative players want to receive — doesn't exist, because there is no space between the lines.

The obvious question is what about the space behind. A high line leaves a lot of pitch between the defence and the goalkeeper, and Sacchi's answer was partly the offside trap, partly a keeper willing to come out, and partly the argument that if the opposition is under pressure they can't play the pass that exploits it. Which is the same argument every high-pressing coach makes today.

The training was the innovation

Here's the part I find most instructive as a coach. The tactical ideas were not, on their own, unprecedented — versions of zonal defending and high pressing existed in the Netherlands and elsewhere. What was genuinely new was the methodology for installing them.

The famous shadow-play sessions, where the team moved through defensive shapes without a ball or against no opposition, were about building an automatic collective response. Not "understand this principle" but "move to this position without thinking about it, ten thousand times." The comparison people always reach for is a dance company or an orchestra, and it's apt.

He was also apparently ruthless about the small distances. Two metres out of position was a stoppage. That level of precision seems obsessive until you consider that a block only works if it's actually compact, and a defensive line where one player is three metres deep is not a line, it's an offside safety net for the opposition.

What didn't survive

Worth being honest that not all of it transferred, and some of it was of its time.

The rigidity, primarily. That Milan side had extremely defined positional responsibilities and relatively little of the fluid rotation that characterises modern possession teams. Positional play as it's now practised — with players interchanging, inverting, occupying zones rather than positions — is a different thing, and arguably a descendant of Cruyff's line of thinking more than Sacchi's.

The offside trap, as I've argued elsewhere, has also largely gone, killed by technology and by changes in how forwards make runs.

And the physical demands of that system on players in an era with much smaller squads and worse sports science were, by most accounts, brutal. It's not clear the model was sustainable over many seasons, which may partly explain why the peak was relatively short.

Why it still matters

The reason to study it isn't nostalgia. It's that the underlying question — should my defenders orient to the ball or to the man — is live again, in a way it hasn't been for twenty years.

The current wave of man-oriented pressing systems is, in a sense, a partial return to what Sacchi argued against, applied in a much more sophisticated form. And the tension between the two approaches, between collective shape and individual responsibility, turns out to be one of the permanent arguments in football rather than a question that got settled in 1989.

What Sacchi established isn't a specific system, then. It's a way of thinking about defending as a collective, coordinated action rather than a sum of individual duties. Whatever the current fashion, that framing is the one every coach now starts from, including the ones arguing for the opposite conclusion. That's about as complete a victory as an idea can have.