Sprint is a "critical part" of SoftBank's plan for the US, SoftBank chief Masayoshi Son said. His company wants to be ready for a world where connected devices are everywhere, and owning a "world class" wireless network is key to making that happen.

The increased stake answers a lingering question: if Sprint couldn't count on the T-Mobile merger, was SoftBank willing to go the extra mile and make sure its American network was in good financial shape? Clearly, the answer is yes. In theory, this gives Sprint more breathing room as it gets its house in order and improves its network to compete with fast-moving rivals.