When AT&T announced its plans to purchase T-Mobile for $39 billion, the most vocal opposition was third-place carrier Sprint. Very early on, Sprint warned U.S. regulators that the acquisition would "alter dramatically the structure of the communications industry" and said that AT&T and T-Mobile combined would be three times its own size. Today, Sprint officially filed a petition with the FCC to block the merger.
Sprint had just testified before a Senate judiciary subcommittee meeting earlier this month, where CEO Dan Hesse claimed that the merger could lead to a duopoly. Of the many sections in the 377-page filing today, one is called "Alice in Wonderland" where Sprint points out that AT&T is playing naive in its attempt to acquire more spectrum when it's currently sitting on available and unused spectrum.
Another section even goes on to detail that AT&T did not adequately prepare its network and that the carrier could have made better use of its spectrum and been more efficient with its resources. In contrast, Sprint has been diligently preparing its network for future consumer demand. Sprint also alleges that AT&T has more than sufficient spectrum capacity but that it's attempting to take a shortcut now that it see's its behind rivals Sprint and Verizon with 4G deployment.
When AT&T filed with the FCC to acquire T-Mobile, they're main argument was that the acquisition was necessary for them to accommodate future consumer demands on their network. The FCC has recently requested AT&T to respond to a list of questions.
You can check out the Sprint's full filing document here.