Two headers by Mario Mandzukic made the difference as Croatia took the lead in Euro 2012's Group C after a 3-1 win over Ireland.
Mandzukic scored the fastest goal of the tournament so far when he opened the scoring in the third minute, with a header from Darijo Srna's deflected ball.
Ireland responded fifteen minutes later with a close-range header from defender Sean St. Ledger, who used his strength to beat Vedran Corluka to Aiden McGeady's free kick.
Everton forward Nikica Jelavic restored Croatia's lead on 43 minutes, flicking the ball over the onrushing Shay Given from Luka Modric's pass.
The Irish claimed the strike was offside, but the referee dismissed their appeals. The match was then stopped for around a minute as Croatian fans celebrated by throwing pyrotechnics.
Mandzukic scored three minutes into the second half as well, with a header from Ivan Perisic's cross that ricocheted in off the post and Given's head.
St. Ledger said he was disappointed by Ireland's defensive lapses.
"It was unlike us to concede the goals we conceded today," he said in comments on the UEFA website.
"Croatia didn’t split us open. From our point of view they were disappointing goals to concede."
After Ireland pulled level, St. Ledger's fellow defender Stephen Ward missed a chance to give Ireland the lead as he failed to connect with the ball to turn in Damien Duff's off-target effort.
On 33 minutes Tottenham midfielder Modric produced a spectacular shot from distances, but it was an easy save for Ireland goalkeeper Given. Two minutes later, a similar attempt by Ivan Perisic missed the target.
After Jelavic's goal, Croatia were centimeters from making it 3-1 before the break as the ball whizzed over the bar from Ivan Rakitic's header.
After Mandzukic extended Croatia's lead early in the second half, Ireland coach Giovanni Trapattoni reacted with a double substitute, bringing on forwards Jon Walters and Simon Cox for McGeady and Kevin Doyle.
Despite the attacking intent, Ireland made little progress while Croatia missed several more chances, including a 78th-minute shot from Rakitic, which went wide.
Ireland fought in vain for a way back into the game in the dying minutes and the five minutes of added time.
On 83 minutes Long's strained for the ball in aerial challenge and his elbow hitting the face of Croatia keeper Stipe Pletikosa, although the referee decided against a booking.
In stoppage time Keith Andrews went close to scoring the match's fourth headed goal, but saw his effort fly wide.
Earlier Sunday, two goals in three minutes saw Spain and Italy share the points with a 1-1 draw in their Euro 2012 opener, as Cesc Fabregas canceled out Antonio Di Natale's strike with a goal of his own.
Ireland: Shay Given, John O'Shea, Sean St Ledger, Stephen Ward, Richard Dunne, Damien Duff, Keith Andrews, Glenn Whelan, Robbie Keane (Shane Long, 75), Kevin Doyle (Jon Walters, 53), Aiden McGeady (Simon Cox, 54).
Croatia: Stipe Pletikosa, Gordon Schildenfeld, Vedran Corluka, Ivan Strinic, Darijo Srna, Ivan Rakitic, Luka Modric, Ognjen Vukojevic, Nikica Jelavic (Niko Kranjcar, 72), Mario Mandzukic, Ivan Perisic.
