Former Beatle Sir Paul McCartney has criticised the UK government for being too hasty in going to war in Iraq.
Sir Paul wrote a song, Freedom, after the 11 September attacks
"Maybe our government went in too fast with the Americans," he told the weekly Portuguese magazine Visao on Thursday.
"It would have been better if the UN had been together," the 61-year-old singer continued. "Now it's become very bloody with Iraq, it's very difficult."
The singer, who is currently touring Europe, opens the Rock in Rio music festival in Lisbon on Friday.
Sir Paul said he understood the need to act after the terrorist attacks of 11 September 2001.
"If someone came to my house and blew it up, I wouldn't just want to sit there and say thank you," he said.
"I'd be angry like I think anyone would be, so I could see America and Britain being angry.
"To look for Bin Laden seemed reasonable to go against terrorism, but the war has become very difficult."
Sir Paul, who became a father for the fourth time last October, wrote a song, Freedom, after witnessing the 2001 attacks on the World Trade Center.
The Rock in Rio festival - modelled on an annual Brazilian festival which has been running since 1985 - is held over the next two weekends.
Britney Spears and Peter Gabriel are among the more than 70 performers taking part in the event, held at Lisbon's Bela Vista Parque.
Sir Paul's performance is part of a European tour which began on Tuesday.