As these examples demonstrate, it is not always easy to define when a violation of antitrust laws occurs. Interpretations of the laws have varied, and analysts often disagree in assessing whether companies have gained so much power that they can interfere with the workings of the market. What's more, conditions change, and corporate arrangements that appear to pose antitrust threats in one era may appear less threatening in another. Concerns about the enormous power of the Standard Oil monopoly in the early 1900s, for instance, led to the breakup of Rockefeller's petroleum empire into numerous companies, including 73

the companies that became the Exxon and Mobil petroleum companies. But in the late 1990s, when Exxon and Mobil announced that they planned to merge, there was hardly a whimper of public concern, although the government required some concessions before approving the combination. Gas prices were low, and other, powerful oil companies seemed strong enough to ensure competition.