30-year-old American Edward Snowden, a contract employee at the National Security Agency, is the whistleblower behind significant revelations that surfaced in June 2013 about the US government's top secret, extensive domestic surveillance programmes. Snowden flew to Hong Kong from Hawaii in May 2013, and supplied confidential US government documents to media outlets including the Guardian.
Spying scandal puts an end to Barack Obama's long European honeymoon
US president faces continent disappointed with achievements and outraged by spying scandal
Explosive revelations about US phone and internet surveillance programs will challenge President Barack Obama's popularity and moral authority when he arrives in Europe today.
Because he was not George W. Bush, who was reviled by millions across the continent, and thanks to a magnetic personal story and rise to power, Obama wallowed in hero worship as a candidate in Europe in 2008 and on debut presidential trips.
Candidate Obama was the prophet of hope, who told 200,000 young people in Berlin pining for a new John F. Kennedy, that Europe and America must remember their destiny and "remake the world once again".
Yet at home and abroad, in the teeth of a global financial firestorm, President Obama learned that delivering change was tougher than promising it.
While he honoured a vow to end the deeply unpopular Iraq war, and will halt Nato combat in Afghanistan next year, Obama has disappointed Europeans on issues like climate change and closing Guantanamo Bay.
And now, he has been revealed as the figurehead of a secret American intelligence war that is more sweeping than anyone knew.
Europeans are among foreigners in the crosshairs of a vast phone and internet surveillance program run by the shadowy US National Security Agency (NSA), now exposed in newspaper leaks.
The man who once vowed to moderate unpopular ex-president Bush's "war on terror" will be the programme's chief defender when he arrives in Northern Ireland for the G8 summit and heads to Germany.
"Certainly, this is not a great PR exercise by the Americans, and this rekindles all of these bad memories of George W. Bush's second term," said Michael Geary, a fellow at the Wilson Centre.
"After four years, when Obama has tried to put a softer face on American power, this is certainly not going to prove very popular for the administration in Europe and for Obama as well."
NSA programs have stirred warnings that the privacy of Europeans is being besmirched by an American Big Brother.
German Chancellor Angela Merkel, raised under the snooping ears of East German government spies, has vowed to bring up the question on the telephone and Prism internet programs with Obama in Berlin.
The White House sought to ease German fears on Friday, saying Obama will explain that NSA programmes do not target individuals or specific conversations, but "meta-data" in a complex equation to snare terrorists.
It also pointedly noted that Germany was a staging point for some of the hijackers behind the September 11 attacks in 2001.
G8 leaders, who know the extent to which European security agencies are co-operating with the NSA, face their own political pressure over the scope of the US spying programme.
"Privately, the Europeans will be extremely vocal on their concerns about this," said Heather Conley of the Centre for Strategic and International Studies. "They will, I think, try to send a more soothing message publicly."