What is the wiretapping case of U.S.? Tell me. Explain me.

I was so sad when I heard the bad news.
"U.S. wiretapped  the conversation of Prime Minister Merkel"
What's that?
Tell me.
Explain me.
U.S. wiretaps all countries secret?
U.S. Government must give more explanations about the wiretapping case of Prime Minister Merkel.
Otherwise, People of the world cannot trust U.S.
Don't leave it.
The matter is very serious.