The world is spinning quickly for Sprint Nextel (NYSE:S). Just days after it accepted Japanese mobile operator SoftBank's offer to buy 70% of it for $20 billion, Sprint has filed with the SEC its intent to buy a controlling interest in Clearwire (UNKNOWN:CLWR.DL).

Once upon a time, Sprint already had voting control of Clearwire but gave that up when it feared becoming entangled in Clearwire potentially defaulting on its debt obligations. At the time, Sprint reduced its slice of voting shares to 49.8% but kept its ownership value in Clearwire at 54%.

This new filing shows Sprint seeking to buy enough Clearwire Class B shares from Craig McCaw's Eagle River Holdings to give Sprint 50.4% of the voting shares. McCaw founded Clearwire in 2003.

So, what's changed?
According to The Wall Street Journal, which cited unnamed sources knowledgeable of the SoftBank/Sprint dealings, the entities putting up the money for SoftBank want guarantees that Sprint would have control of Clearwire.

Money-losing Clearwire is attractive to Softbank for a couple of reasons. First, both companies' high-speed networks now under construction use similar technology, and, second, Clearwire sits on a large cache of invaluable wireless spectrum.

But Clearwire has been incurring a heavy debt load as it rushes to join the 4G LTE networking club and it is imperative Sprint and Softbank have the power to say no to Clearwire raising money by selling off that spectrum – especially to AT&T (NYSE:T) and Verizon (NYSE:VZ).

Earlier in the week, Clearwire shares shot up 26% on the hopes that Sprint would completely take it over. On news that something less than a total acquisition will happen, Clearwire's shares have fallen back to earth -- and then some -- 11% lower than where it started on its climb up.

Fool contributor Dan Radovsky owns shares of AT&T. The Motley Fool has no positions in the stocks mentioned above. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.