Just as U.S. President Joe Biden is set to travel to Europe later this month for the Group of Seven summit of world leaders in Cornwall, UK, fresh reports revealed that the world's No.1 superpower had been spying on its longtime allies in the European continent, again.

The U.S. National Security Agency (NSA) has used a partnership with Denmark's foreign intelligence unit to spy on senior officials in Germany, France, Sweden and Norway from 2012 to 2014, Danish state broadcaster DR reported on Sunday, citing nine unnamed sources with access to the investigation.

European leaders including German Chancellor Angela Merkel, French President Emmanuel Macron and Swedish Defense Minister Peter Hultqvist have voiced criticism, calling the reported eavesdropping "unacceptable" and demanding answers from the U.S.

But they, as well as other countries, might not be so shocked by the renewed spotlight on the tapping by the U.S., which is notoriously known for blatant espionage activities using its leading-edge technologies for a long time. 

Snowden leaks in 2013

The uproar could easily bring back Europeans' memories about the dark days in 2013, when whistleblower Edward Snowden disclosed numerous global surveillance programs run by the U.S., especially on its European allies.

The former contractor for the U.S. Central Intelligence Agency (CIA) revealed that the NSA was not only collecting the telephone records of tens of millions of Americans on an "ongoing daily basis," but also tapping the mobile phones of 35 heads of state – including that of Angela Merkel.

The U.S. bugged Merkel's phone from 2002 until 2013, and also spied on millions of phone calls, emails, and text messages of ordinary German citizens, according to a report in Der Spiegel magazine.

After the German chancellor called then U.S. President Barack Obama and voiced strong discontent, the latter apologized and promised to halt the operation.

The latest report just proved that the U.S. did not honor its words since the espionage continued to at least 2014.

Other explosive secrets revealed by Snowden include:

The U.S. tapped directly into the servers of nine internet firms, including Facebook, Google, Microsoft and Yahoo, to track online communication in a surveillance program called Prism, which enabled analysts to track foreign targets.

The NSA paid its UK counterpart GCHQ to tap fiber-optic cables that carry global communications, including email messages, Facebook posts and internet histories, which then shared the data with the NSA. The operation codenamed Tempora had been running for 18 months when disclosed by Snowden.

The GCHQ also intercepted phone calls and online communications of international politicians attending two G20 meetings in London in 2009 and shared the intelligence with the U.S.

The NSA and GCHQ eavesdropped on Italian phone calls and internet traffic and targeted three undersea cables with terminals in Italy.

The U.S. bugged the European Union's offices in New York, Washington, and Brussels.

The U.S. had monitoring stations in Australia and New Zealand that helped feed data back to the NSA.

The U.S. spied on at least 38 foreign embassies and missions, and the targeted countries include France, Italy, Greece, Japan, South Korea and India, all its allies.

The NSA conducted a continent-wide surveillance program on citizens in a number of Latin American countries, including Brazil, Mexico, Venezuela, Colombia, Argentina, Panama, Ecuador, Peru and Chile.

The NSA had led more than 61,000 hacking operations worldwide, including many in the Chinese mainland and Hong Kong, with universities, public officials and businesses being targeted. 

Revelations by WikiLeaks in 2015

In 2015, the whistleblowing website WikiLeaks brought to light more spying activities by the U.S. with solid documents. Countries including Japan, France and Germany were targeted.

The U.S. NSA was spying on Japanese cabinet officials, banks and companies, including the Mitsubishi conglomerate, which extended back at least eight years.

The agency had collected information on Japan's internal affairs, including trade talks, climate change policy and nuclear policy.

It shared the intelligence with the so-called "Five Eyes" group, including Canada, the UK, New Zealand and Australia.

During a period of at least 2006 until May 2012, the U.S. tapped three former French presidents – Jacques Chirac, Nicolas Sarkozy and Francois Hollande, as well as French cabinet ministers and the French ambassador to the U.S.

WikiLeaks also revealed that the U.S. tapped German chancellery for decades. Not only had the NSA's spying on Merkel and her staff gone on far longer and more widely than previously realized, but staff for Merkel's two predecessors, Gerhard Schroeder (in office 1998-2002) and Helmut Kohl (in office 1982-1998), were also under surveillance for a long time.

'To be America's friend is fatal'

Former U.S. Secretary of State Henry Kissinger once said: "it may be dangerous to be America's enemy, but to be America's friend is fatal."

Considering the long yet incomplete list of U.S. spying scandals, the latest revelation by Danish media could still be the tip of the iceberg.