In a filing today with the SEC, Sprint Nextel Corp. (NYSE: S) announced that it is making an offer to buy the 740 million shares (49%) of Clearwire Corp. (NASDAQ: CLWR) it does not already own for a cash price of $2.90 a share. That is a 5.5% premium to Clearwire’s closing price last night. The total value of the deal is approximately $2.15 billion.

Sprint, already the object of a $20 billion takeover, must get approval from its prospective new owner — Japan’s Softbank — before entering into a definitive agreement to acquire the rest of Clearwire.

The only conceivable reason for this acquisition is that Softbank and Sprint had an understanding at the time of Softbank’s offer for Sprint that the U.S. phone company would either fish or cut bait with Clearwire. In the end, the temptation of Clearwire’s spectrum holdings proved just too enticing.

Sprint’s shares are trading down about 1.2% in the premarket this morning, at $5.66 in a 52-week range of $2.10 to $6.04. Clearwire’s shares are up 10.9% to $3.05, a new 52-week high if it holds.

Paul Ausick