Up front, Button was immediately under threat from Alonso. This was not only because of Button's high downforce set-up, but because contact between him and Alonso in turn one on the first lap had damaged Button's rear diffuser.
Button was easy prey for Alonso on the straights, but he did not make any mistakes in the corners and was able to hold him off.
After 10 laps Button put in a series of quick laps and briefly broke away, but Alonso soon reeled him in again.
On lap 21 Vettel ceded seventh place to Webber, telling the team over the radio that his engine was damaged.
Button pitted first freeing Alonso to push hard in free air. The Ferrari driver pitted a lap later and emerged in the lead.
From then on Alonso was never headed and fully deserved to win in front of the ecstatic Italian fans.
Massa finished third with Vettel fourth. The Red Bull driver's engine troubles had seemingly resolved itself and he made his mandatory pit stop on the last lap to leapfrog a frustrated Webber and Nico Rosberg in the Williams.
The result makes the drivers' championship even closer. Webber provisionally leads with 187 points, Hamilton is second with 182, Alonso is third with 166, Button is fourth with 165 and Vettel in fifth has 163.
Provisional race results
Autodromo di Monza, Italy; 53 laps; 306.720km;
1. Alonso Ferrari 1h16:24.5722. Button McLaren-Mercedes +2.9383. Massa Ferrari +4.2234. Vettel Red Bull-Renault +28.1935. Rosberg Mercedes +29.9426. Webber Red Bull-Renault +31.2767. Hulkenberg Williams-Cosworth +32.8128. Kubica Renault +34.0289. Schumacher Mercedes +44.94810. Barrichello Williams-Cosworth +1:04.20011. Buemi Toro Rosso-Ferrari +1:05.0012. Liuzzi Force India-Mercedes +1:06.10013. Petrov Renault +1:18.90014. De la Rosa Sauber-Ferrari +1 lap15. Alguersuari Toro Rosso-Ferrari +1 lap16. Sutil Force India-Mercedes +1 lap17. Glock Virgin-Cosworth +2 laps18. Kovalainen Lotus-Cosworth +2 laps19. Di Grassi Virgin-Cosworth +2 laps20. Yamamoto HRT-Cosworth +2 laps
Fastest lap: Alonso, 1:24.139
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