O'Neill's men left to regret not meeting their own standards as Northern Ireland lose out to Poland
After the glory of qualification comes the reality check after Milik strikes to sink Northern Ireland
Northern Ireland fans had waited 30 years for this. They had dared to dream but once the moment and the match came the reality hit hard.
And so did the performance and the outcome.
Bottom line Northern Ireland weren't good enough on the day the country made history by playing in the European Championship finals for the first time and in their first tournament since the 1986 World Cup.
The biggest disappointment for the manager Michael O'Neill, his players and the supporters will be that they know they are better than this.
The scoreboard inside Stade de Nice read 1-0 to Poland at the final whistle, but the truth is the teams were Poles apart.
O'Neill's side entered the tournament on a record 12 match unbeaten run. There was nothing unlucky about the defeat in game number 13.
Poland deserved their victory. They were the superior team throughout. From the first few seconds they took control and did not let go. Arkadiusz Milik's strike in the 51st minute proved enough for them to start Euro 2016 with a bang.
Had the Poles been more clinical or had more of their chances fallen to ace marksman Robert Lewandowski they probably would have won by more.
Sure, Northern Ireland showed the grit, heart, togetherness and 100% commitment to the cause we expect but then so did Poland and they had more ability and adventure to go with those other qualities.
For 20 months just about everything that O'Neill has touched has turned to gold with Northern Ireland topping their qualifying group and becoming invincible. Arriving in France the manager had the players on a high believing anything was possible only for things to come crashing down to earth with a bump yesterday.
As O'Neill inspired Northern Ireland to France everyone seemed comfortable with their roles.
In Nice there was only discomfort as they came under pressure from early on.
O'Neill's team selection was both fascinating and surprising. Michael McGovern was in goal with a back five of Conor McLaughlin, Craig Cathcart, Jonny Evans, Gareth McAuley and Shane Ferguson. There was a midfield diamond with the boss hoping Chris Baird, Paddy McNair, Oliver Norwood and captain Steven Davis would sparkle.
Essentially Davis was operating behind lone striker Kyle Lafferty. No Jamie Ward or Stuart Dallas. Both could consider themselves unlucky.
It looked like a defensive set-up and that's how it played out during an agonising first 45 minutes for the vociferous Northern Ireland fans and a serious test for the players. Poland were dominant in possession but in promising positions too often their final ball let them down.
In a bid to gain more control O'Neill swapped Davis with McNair, but on the rare occasions they had possession Northern Ireland failed to do anything worthwhile with it.
Milik, who had fluffed two chances early on, missed an even better opportunity following the half hour mark slicing horribly wide after the Northern Ireland defence had been sliced apart.
The next moment of concern for O'Neill's side came when Bartosz Kapustak's effort was brilliantly tipped over by McGovern.
O'Neill continued to change tactics with Northern Ireland finishing the opening period in what appeared to be a 4-5-1 formation with Lafferty isolated and totally out of the game.
On the plus side they weren't behind as defenders Evans, McAuley, Cathcart, Ferguson and McLaughlin stood as firm as they could in the face of such an onslaught.
Dallas replaced McNair at the break to add more balance and width. You had to feel for 21-year-old McNair, a wonderful prospect, being asked to play in several different positions and in particular just behind Lafferty. He's versatile, but in a game of this magnitude that was too much to ask.
Within six minutes of the interval, Northern Ireland were behind. Jakub Blaszczykowski was allowed the freedom of Nice to cross from the right and with Milik, in space inside the area, he calmly slotted past McGovern as Cathcart and McAuley tried to block.
Conor Washington entered the fray and finally Northern Ireland looked threatening. The energetic Ward soon followed him on to the pitch.
Even so Poland were still the more likely team to score.
Northern Ireland's only real chance came in the dying minutes. Their set pieces had largely been a let-down until Norwood's smart pass picked out an equally clever run from the captain, who was inches away from connecting with the ball just yards from goal with 10,000 fans in green ready to erupt and hail a late leveller.
It wasn't to be.
Disappointment but not total despair. The dream is not over yet. It's Lyon next on Thursday against Ukraine when O'Neill's men simply have to win.
Hopefully the real Northern Ireland will show up then.