“Delta IV Heavy” the world’s most powerful rocket is belong to which country?

United States
Recently, the United States has successfully launched its secret spy satellite “NROL-37” to space by the United Launch Alliance’s (ULA) Delta IV Heavy rocket, the world’s most powerful rocket, at Florida’s Cape Canaveral Air Force Station. The National Reconnaissance Office (NRO) is responsible for designing, building and operating the United States’ fleet of spy satellites.