The basic answer, some national intelligence and security experts say, is that relations among countries are essentially based on interests, and no matter how friendly countries may be, their interests are rarely exactly the same.

“We and Germany don’t always see eye-to-eye on some important issues,” says James Lewis, director of the Technology and Public Policy Program at the Center for Strategic and International Studies in Washington. “One way to reassure yourself about the direction an ally like Germany is heading on one of those issues is to know what Germany is saying.” …

With intelligence capabilities expanding in ways unimagined just a matter of years ago, “It becomes an accretive growth process,” says Allan Friedman, research director of the Center for Technology Innovation at Washington’s Brookings Institution.

Asked how this process could end up with Merkel’s cell phone being “tasked,” Mr. Friedman says, “The prevailing thinking is, ‘We got that much [information] this year, what can we do to get more next year?’ ”