Regardless of the result, this felt like a fitting end to the campaign – a match between two sides lining up with a three-man defence. It had, of course, been a 3-0 defeat at the Emirates in September which encouraged Antonio Conte to switch to his 3-4-3 system, and then Chelsea’s astonishing run of form seemingly encouraged Arsène Wenger to follow suit.

For all the pre-match speculation about Arsenal’s defensive problems and the controversial decision to play David Ospina rather than Petr Cech, Wenger’s key decision here was up front. For the 4-0 FA Cup final victory over Aston Villa two years ago, Wenger elected for the speed of Theo Walcott and omitted Olivier Giroud, and the extra pace forced Aston Villa’s defence to play deeper, creating space between the lines when their midfield tried to press, allowing Alexis Sánchez and Mesut Özil to run riot. Here, an almost identical thing happened – albeit with Danny Welbeck, rather than Walcott.

Welbeck was excellent throughout the first half, making dangerous runs into the channels and terrifying Chelsea’s backline with his speed. He played up against David Luiz, but made runs into wider positions, winning a foul from the usually unflappable César Azpilicueta when running in behind and later dragging Gary Cahill out of position for Özil’s good chance in the inside-right position.

But more than anything, it was the threat of Welbeck’s speed in behind that proved crucial. Chelsea started the game incredibly deep, surely much deeper than had they been playing against Giroud, which allowed Arsenal time on the ball in midfield. Sánchez’s opener was controversial, but it’s not often you feel a fourth-minute goal had been on the cards; Arsenal had started superbly.

Perhaps Chelsea wanted to play on the counterattack, seeking to expose Per Mertesacker’s lack of speed. But such a reactive strategy was less viable when 1-0 down so early. Conte came running out of his dugout to gesture for his players to push up the pitch, and centre-forward Diego Costa made similar motions. But Chelsea looked extremely reluctant to press, and the positioning of their defensive line barely changed. When N’Golo Kanté and Nemanja Matic pushed forward, they were generally too slow to close down Aaron Ramsey and Granit Xhaka, who could prod the ball through the lines to Sánchez and Özil in space. Arsenal were dominant, storming forward on the counterattack and forcing a couple of set pieces, with Welbeck and then Ramsey hitting the post in quick succession, then Welbeck having a good chance on the left.

The onus was on Chelsea to get back into the game, and their first-half performance was so poor that you suspected Conte might change things even before half-time. Instead, he waited until the hour mark before inevitably summoning Cesc Fàbregas, whose absence from the Chelsea midfield in the first half exposed a lack of creativity in that zone. But Fàbregas barely had a chance to play his way into the game before Victor Moses received his marching orders with a second bookable offence for his dive in the Arsenal penalty area.

This was a new problem for Conte – the only previous red card shown to a Chelsea player this season was John Terry’s with Chelsea 3-0 ahead against Peterborough in the third round. Reshaping from a 3-4-3 isn’t simple, and so Chelsea ended up moving to a 4-3-1-1, with Azpilicueta sliding across to right-back. No one was out of position, but having played with a back three all season, Chelsea’s shape simply did not feel right.

Yet they got themselves back into the game through a very simple route – substitute Willian’s deep cross into Costa, who fired home via a Mertesacker deflection. But Arsenal’s unfamiliar backline had largely stood firm: Nacho Monreal made some excellent interceptions, Mertesacker was absolutely outstanding considering this was his first start for over a year, and young Rob Holding was confident enough to dish out some verbals to Costa in the opening stages, a moment that suggested Arsenal, for once, were not going to be bullied by Chelsea.

Chelsea were level for only two minutes, however. Welbeck had tired and was replaced by Giroud immediately after the equaliser. And while the Frenchman is less inclined to run in behind the opposition defence, that’s precisely what he did here, spinning into the left-hand channel before playing a blind – but perfectly-weighted – chip into the box, met by Ramsey’s typically well-timed run and simple header. Wenger is rarely praised for his tactical acumen, but he got his decision-making up front absolutely spot on, both with his initial selection and timing of the substitution.

In a wider context, too, Wenger has rarely demonstrated his strategic ability so impressively. His switch to a three-man defence prompted a run of nine wins from 10 matches, ending with this confident victory over the best side in the country. Wenger declared it one of the proudest victories of his Arsenal career, and in a tactical sense, considering his injury crisis in defence, it was perhaps the most impressive.