Apple's relationship with Ireland, where it receives much of its profit, was always a mystery: The world's most valuable company had clearly made some sort of deal to pay as little tax as possible, but the details of that deal had never been revealed — until now. Revealing as they are about Ireland's practices, they require action primarily from the United States.
More than three months after the European Commission started investigating the possibility that Ireland had been providing illegal state aid for Apple, it finally published a letter containing the information that aroused its suspicions.
Essentially, in 1991 and 2007, Apple and Ireland agreed that much of the money flowing into the company's Irish accounts had nothing to do with its operations in the country, and hence shouldn't be taxed as profit in Ireland.