John Legere, CEO and President of T-Mobile USA, makes an announcement during an event about new contract pricing on March 26, 2013 in New York City. (Photo by John Moore/Getty Images)

That was quick.

It wasn't long ago that Sprint and T-Mobile were supposed to be linking up. Then — though not to analysts' surprise — the deal fell through in the face of regulatory skepticism. Now, T-Mobile has turned right around and is now trying to undermine Sprint by stealing away its customers.

As part of a new promotion, T-Mobile is offering free unlimited LTE data to customers who convince Sprint users to switch. The promotion applies both to the referrer as well as to the Sprint customer coming over to T-Mobile.

T-Mobile's offer follows a promotion launched by Sprint this week that overhauls Sprint's pricing. On Thursday, Sprint also announced a new unlimited plan that saves customers money without "jump[ing] through T-Mobile's hoops," as Sprint spokeswoman Crystal Davis put it. Those deals are aimed at all the major national carriers, not just T-Mobile.

In keeping with its snarky image, T-Mobile is marketing its promotion as a way for its customers to "rescue" Sprint users.

It looks like there's no love lost between the two companies. And that makes sense; the wireless market is so heavily saturated as to be a zero-sum game for post-paid customers at this point. Still, it's remarkable how quickly these two companies have turned on one another.