With the memory of their Wembley masterclass against Manchester United still fresh, Barcelona begin a new Champions League campaign on Tuesday with a mouth-watering Group H opener against Italian thoroughbreds AC Milan.Barca, kings of Europe four times compared to the seven triumphs of their opening opponents, will start as favourites to become the first club to retain the trophy since Milan achieved the feat in 1990.Lionel Messi, top scorer in the competition for the past three season, will be setting his sights on Milan for the first time, having never faced them in a competitive match.While keeping an eye on Messi, Milan will also be hoping to capitalise on the defensive insecurity that resulted in Barca dropping two La Liga points in a 2-2 draw at Real Sociedad at the weekend."It's not easy to get back to what we do well, but we can't let it happen again," defender Sergio Busquets told Barca's website (www.fcbarcelona.com). "They are the champions of the Italian league and we mustn't forget that".Chelsea's pursuit of Europe's big prize continues as they begin Group E at home to Bayer Leverkusen while in another Anglo-German clash, Arsenal travel to Bundesliga champions Borussia Dortmund.Competition newcomers Viktoria Plzen, the surprise Czech champions, will be targeting a home victory against Belarus side BATE Borisov before they turn their attention to more daunting fixtures against Barcelona and Milan.