The merger of Sprint (NYSE:S) and T-Mobile (NASDAQ:TMUS) has seemed inevitable, but there's a new wrinkle that could threaten to derail the combination.

Sprint and T-Mobile have been in talks for over six months, with Masayoshi Son, the founder and CEO of SoftBank -- Sprint's  majority owner -- being very open about his desire to combine the carriers.

Now it appears Sprint has a rival attempting to steal T-Mobile out from under its nose. French telecom operator Iliad said it offered $15 billion in cash for 56.6% of T-Mobile U.S., The Wall Street Journal reported. 

That may make it a contender, but it's less than the $32 billion value that Sprint, the No. 3 wireless carrier, puts onto No. 4 carrier T-Mobile. Though no deal has been reached, Sprint and T-Mobile have agreed on the broad parameters of a merger. It's likely that the Iliad offer is not high enough, but it's not so far off that it won't be seen as credible. It's also important to note that while Iliad is a player elsewhere, it does not operate in the U.S., so it's unlikely to face the same regulatory hurdles that Sprint would in making the acquisition. 

"The U.S. mobile market is large and attractive," Iliad told WSJ. "T-Mobile US has successfully established a disruptive position, which in many ways, is similar to the one Iliad has built in France."

Iliad is known for causing a price war in the country's mobile phone market. T-Mobile is currently attempting to do the same thing -- with strong results -- in the U.S.

The Iliad offer may not be good enough, and there is doubt as to whether the company could even finance it. But the looming specter of an alternate suitor may push Sprint to act faster. 

Why does everyone want T-Mobile?
Sprint wants T-Mobile in order to have the scale in the United States to take on the top two players in the field, AT&T and Verizon. As separate companies with much smaller subscriber bases than their rivals, Sprint and T-Mobile are at a competitive disadvantage. They still have to maintain nationwide networks, sales forces, marketing teams, and retail stores. Because they are not as large, those costs are amortized across fewer users.

If you combine the companies, it becomes possible to spend less while still investing in network improvements. The cost efficiencies created by a merger are obvious and far-reaching.

That may not impress U.S. regulators who have said they are opposed to seeing the number of major U.S. wireless carriers drop from four to three.

For Iliad, the benefits of the deal seem less obvious, but some of the same principles apply. Iliad founder Xavier Niel said a merger would result in $10 billion in synergies and an additional $2 billion in annual earnings.

Analysts, however, are skeptical that those numbers are realistic.

"T-Mobile is not bloated at all. It is cut to the bone," Roger Entner, an analyst at Recon Analytics, told "T-Mobile already has a history of squeezing the vendors. But you can't squeeze water out of a stone."

Cutting your way to prosperity has historically not been a successful long-term strategy. There are major holes in the Iliad plan, but that does not mean a deal can't be worked out.

Why is T-Mobile for sale?
T-Mobile has been crushing it since CEO John Legere introduced the "Un-Carrier" strategy that markets the company as different than its competitors. T-Mobile added 1.5 million subscribers in the second quarter, the fifth consecutive quarter with more than 1 million net customer adds. T-Mobile has added 7.6 million new customers over the past 15 months.

That growth has come from aggressive pricing and a more transparent customer experience.

T-Mobile has also been building out its network. Its 4G LTE network covers more than 233 million people in 325 metro areas. The company is rapidly deploying Wideband LTE and will be rolling out its 700 MHz A-Block spectrum beginning this quarter. In addition, T-Mobile is the first company to achieve nationwide voice-over-LTE, or VoLTE, coverage, covering more than 200 million people.

"The improvements to increase speed, capacity, and coverage across the T-Mobile network footprint are rapid and ongoing," wrote the company. 

All of these things cost money, however. In 2013, capital expenses pushed the company to an overall loss. In the second quarter of 2014, cash capital expenditures were $940 million, down from $947 million in the first quarter of 2014 and $1.1 billion in the second quarter of 2013. The company may not need a merger in order to have the money to compete, but being part of a larger entity certainly would make things easier. 

Sprint's still the best suitor
Despite the Iliad offer, the best possibility for T-Mobile is a merger with Sprint -- if a deal can be reached with U.S. regulators. While the Iliad cost savings seem a bit fantastical, the ones created through a Sprint merger are much easier to visualize. An Iliad deal leaves a heavily leveraged company, while one with Sprint creates a legitimate third player in the U.S. 

Update: The Wall Street Journal reported Tuesday night that Sprint's board has decided to stop pursuing T-Mobile. The story speculates that the merger may be taken up again when regulatory climate is less hostile.