The National Security Agency has never said what it was seeking when it invaded the computers of Petrobras, Brazil's huge national oil company, but angry Brazilians have guesses: the company's troves of data on Brazil's offshore oil reserves, or perhaps its plans for allocating licenses for exploration to foreign companies. Nor has the N.S.A. said what it intended when it got deep into the computer systems of China Telecom, one of the largest providers of mobile phone and Internet services in Chinese cities. But documents released by Edward J. Snowden, the former agency contractor now in exile in Russia, leave little doubt that the main goal was to learn about Chinese military units, whose members cannot resist texting on commercial networks. The agency's interest in Huawei, the giant Chinese maker of Internet switching equipment, and Pacnet, the Hong Kong-based operator of undersea fiber optic cables, is more obvious: Once inside those companies' proprietary technology, the N.S.A. would have access to millions of daily conversations and emails that never touch American shores. The [US] government does not deny it routinely spies to advance American economic advantage, which is part of its broad definition of how it protects American national security. While the N.S.A. cannot spy on Airbus and give the results to Boeing, it is free to spy on European or Asian trade negotiators and use the results to help American trade officials - and, by extension, ... American industries.