Government security officials had no quick answer Friday for that mortifying question, amid calls in the media for officials to be fired and scathing criticism from security experts on every aspect of Thursday night's near-disaster.
Video and pictures from The Associated Press showed it all. Angry student protesters, pumped up from earlier scuffles with police, found a delectable target elsewhere: Prince Charles and his wife Camilla, sitting in a vintage Rolls-Royce on their way to a charity event at a London theater.
Some protesters chanted "Off with their heads!" Others smashed one of the car's rear windows and splashed it with white paint. An AP picture showed Charles and Camilla visibly shaken but unharmed.
Buckingham Palace does not comment on royal security procedures, but security experts said the prince was lucky to have escaped unharmed. They also identified a host of security failures surrounding the royal outing - and warned that procedures must be radically improved before Prince William's wedding to fiancee Kate Middleton at London's Westminster Abbey on April 29.
"It wasn't potentially dangerous - it was dangerous," said security analyst Charles Shoebridge, calling the attack "one of the most serious security breaches of the past decade."
He said the royal couple should have taken a different route in central London to the theater, or waited until the streets were safe and clear of protesters or simply sent their regrets and canceled.
"The best means of preventing a subject being attacked is to keep him out of harm's way in the first place," he said.
British police should have been talking with the royal protection squad to ensure the couple never came near the protests - and most certainly not in a vintage Rolls-Royce, said Alex Bomberg, a former aide to the royal family and CEO of Intelligence Protection International, Ltd.
The prince's boxy car lacked speed and maneuverability and had big clear windows with reinforced but not bulletproof glass. With two police motorcycles in front and an official royal Jaguar behind it, the vehicle was instantly recognizable as a royal car.
"You couldn't get away in an emergency in a vintage 1977 Rolls-Royce," Bomberg said. "They should have used something more high powered and up-to-date."
Somehow, protesters also managed to get between the police escort and the royal car.
London's theater district is a maze of narrow one-way streets and constantly crowded with tourists and theatergoers. Cars and taxis making their way through the area often crawl at a snail's pace - providing an easy target even for attackers on foot.
The Palladium Theater is on a one-way street. Without a clear escape route, the vehicle and route should never have been used, Bomberg said.
"You can't blame the royal protection squad for a bunch of anarchists' bad behavior," Bomberg said. "But you can blame someone for not doing their job correctly and not understanding the situation as it was unfolding. Someone's head should bloody roll."
Bomberg said police, using live video feeds, should have kept the royal protection squad appraised of the volatile situation and been ready to change plans at a moment's notice.
Although riot police were used to try to corral students in Thursday's protests, very few of Britain's police carry firearms. The prince was accompanied by a security detail with guns - and the situation could have escalated quickly, Shoebridge said.
The chief of the Metropolitan Police pledged to investigate and British media raised questions whether the country needs to ramp up its security measures ahead of the royal wedding. But experts noted that vast resources were already being spent protecting British royals, and this time, the failure was in the execution.
"Existing security resources need to be properly used and existing security protocols to be properly followed," said Shoebridge.
Given the number of world leaders, royals, celebrities, tourists and gawkers expected to descend on London for the royal wedding, he said the police investigation into Thursday's clash needs to be speedy and detailed so changes can immediately be put into place.
"If there is to be any silver lining, it would be that this incident provides a wake-up call to Scotland Yard to learn from this and ensure that the royal wedding passes off trouble free," Shoebridge told The Associated Press.
The Metropolitan Police said it's too early to comment on measures for the royal wedding but that officials were already working on a security plan for the event.
It said the force had launched a "major criminal investigation" into the violence on Thursday.
"It will focus on all of the circumstances behind the violent disorder and look to identify those responsible," the force said.
In previous royal security breaches, Princess Anne escaped a failed kidnapping attempt in 1974 and a teenager shot off six blank rounds at Queen Elizabeth II as she rode by on horseback in a 1981 incident. In 1982, the queen woke up to find a strange man perched on her bed in Buckingham Palace but safely summoned security.
In 1994, a student charged at Charles while firing a starting pistol during a ceremony in Sydney, Australia, and a comedian dressed as Osama bin Laden gatecrashed Prince William's 21st birthday party at Windsor Castle in 2003.
Prime Minister David Cameron said police must learn from the incident but stopped short of blaming the police.
"Let's be very clear about where responsibility lies," said Cameron, speaking Friday in front of his office at Downing Street. "Responsibility for smashing property, or violence, lies with the people who perpetrate that violence and I want to see them arrested and punished."
Metropolitan Police Chief Paul Stephenson commended his officers for their bravery and said the nearly 3,000-strong contingent handling the student protests showed restraint in dealing with "thugs."
Police said 33 protesters were arrested but would not say whether any of the arrests were linked to the royal attack. Scotland Yard said most of those arrested had been released on bail.
Separately, the Independent Police Complaints Commission, a police watchdog, said it would investigate the case of Alfie Meadows, a 20-year-old student who needed emergency surgery for bleeding on the brain after being injured during Thursday's protests. His mother said he had been hit by a police truncheon.
He was among more than 40 students and 12 police officers injured during the protests.