MILAN -- And suddenly, they looked all grown up; freed from the insecurities, brain farts and inferiority complexes that so often derailed them. Inter Milan's 1-0 win over AC Milan amounts to a 3-0 aggregate win over their crosstown rivals and opens the door to the UEFA Champions League final in Istanbul on June 10. It also showed how this club -- whose history of bad decisions, unforced errors, psychodrama, chaos and squandered opportunities is etched into its DNA -- could mature into something greater than the sum of its parts and actually play with confidence and control.

That last part matters. Because even when Inter were really good in the past, even when they were winning the Treble under Jose Mourinho and were a far better side -- in terms of both talent and execution -- they rarely showed the sort of self-assurance they've showed in this Champions League campaign.

This is where critics and pedants will point out that the 2010 Treble winners faced a higher caliber of opponent in the knockout rounds -- Chelsea in the round of 16, Barcelona in the semis and then Bayern Munich in the final -- whereas this Inter side dispatched FC Porto (by the skin of their teeth), Benfica and a stumbling Milan, and won't actually play an A-lister until the final. Very true, but this isn't about Inter being good. It's about Inter being the best they can be, treating warts as if they were beauty spots, masking weaknesses with intelligence and not living every imminent setback as if it were a harbinger of doom. In other words, it's about being mature.

- Stream on ESPN+: LaLiga, Bundesliga, more (U.S.)

Twelve minutes into Tuesday's game at San Siro, Theo Hernandez ripped the ball from Nicolo Barella, leaving the Inter midfielder prone on the turf and 60,000 Nerazzurri in the stands screaming for a foul. That run of play ended with Sandro Tonali smoking Henrikh Mkhitaryan, bursting into the box and delivering a perfect cutback for Brahim Diaz on the penalty spot. It looked like a goal that was harder to miss than to score, but the Real Madrid loanee somehow scuffed it with his wrong foot and it dribbled innocuously into the arms of Inter keeper Andre Onana.

The blue-and-black majority at the stadium breathed a collective sigh of relief before fretting that it would be a sign of things to come, that Milan had their number, that they'd be ripped to shreds like this all night.

The blue-and-black on the pitch? Not so much. They took it in stride, got on with it and played their game. Maybe they had all read Eckhart Tolle's "Power of Now." Maybe manager Simone Inzaghi delivered extracts from it in his pregame speech: "You can't change the past because it's gone, you can't affect the future because it's not here yet, all you really do is shape your present." Whatever the case, it would end up being the only shot on target that Milan would record all night long.

In fact, Inter limited the opponents to just five shots overall, while liberally taking attempts (15 in total) and winning the expected goals (xG) battle (0.91 to 0.54) along the way. This had all the pageantry and choreography of a derby, but the tension on the pitch seemed primarily heaped on Milan's shoulders. Which, in some ways, is odd, given the cliché that when you're 2-0 down and chasing the game, you have very little to lose. Whereas, for Inter, the feat of making a mistake, the fear of screwing it up so close to the finish line, would have followed their age-old script.

A lot of this has to do with a veteran midfield that knows how to make their experience count. Mkhitaryan (and then Marcelo Brozovic, after the former Armenia international went off due to injury), Hakan Calhanoglu and Barella won out against Milan's undermanned duo of Tonali and Rade Krunic (would it have been a different story with a fit Ismael Bennacer? We'll never know). They may not have the legs to cover every blade of grass (and whoever they face in the final, they'll be stretched) but, certainly in the case of Mkhitaryan and Calhanoglu, they play with the abandon of attacking players determined to show they can do the blue-collar stuff too.

This is also where you have to credit Inzaghi. His opposite number, Stefano Pioli, evidently believed they would sit deep and hit on the counter, which would explain why, with Bennacer out, he opted for a 4-2-3-1 featuring the more languid and creative Junior Messias out wide, rather than the speedy Alexis Saelemaekers.

But Inter didn't sit deep.

They pushed on and tried to play out from the back when they could. There would be no siege. They trusted their rearguard, even though it's worth noting that, other than Alessandro Bastoni (one of the better central defenders in Europe), none of these guys were expected to be starters at the beginning of the year. Matteo Darmian was a recycled full-back turned utility player. Onana was the reserve keeper who had hardly played last season at Ajax. Francesco Acerbi was a 34-year-old cancer survivor signed on a free transfer as cover for the extra cover. Onana and Bastoni are legitimately very good and the other two are experienced and gutsy, and sometimes that matters more.

Inter bottled up the bogeyman Rafael Leao, back from the muscular injury that kept him out of the first leg, with wing-back Denzel Dumfries (unquestionably the finest footballer named for a Hollywood leading man). Edin Dzeko dragged his 37-year-old body to join Barella in dampening Theo's runs from deep. That's the bulk of Milan's attacking threat nullified right there. It left Tonali fighting windmills in the middle of the park.

Inzaghi's trademark substitutions did the rest. A fresh Romelu Lukaku coming on in the last 25 minutes to run at (and into) your defenders is about as welcome a sight as blue lights flashing behind you when you've been playing fast and loose with the speed limit.

Most of all, Inter were not fazed. Not by the occasion, not by the tactical fouls (it got tetchy, as derbies do), not by their history. Heck, maybe they told themselves that, for all of Milan's European pedigree with the seven European Cups, it is the Nerazzurri who were in a Champions League final more recently (2010 vs. 2007).

They may well get smoked in Istanbul, whether it's by Treble-chasing Manchester City or by defending champions Real Madrid, but they're playing with house money anyway. Whatever happens, it's highly unlikely they'll beat themselves in Istanbul, like so many of the Inter teams before them. They're too grown up for that.