Sprint is about to become the latest casualty of the US telecom wars — and it’s partly thanks to a temperamental, impulsive Japanese billionaire.

When Sprint was acquired by Japan-based SoftBank in 2013, it was the nation’s No. 3 carrier, boasting cutting-edge tech that included a trove of spectrum that analysts saw as a formidable threat to Verizon and AT&T.

This week, the 28-year-old company got a final thumbs-up to be acquired by former No. 4, T-Mobile, which Sprint had been bigger than in market value at the time when SoftBank acquired it. In the years since, Sprint has shed 10,000 employees, watched its stock seesaw, and gained a reputation for hopelessly spotty service.

What went wrong? Insiders and experts point a finger at SoftBank’s Chief Executive, Masayoshi Son. Brash, impatient and staggeringly wealthy, Son was bent on merging Sprint with T-Mobile as soon as he could. Ignoring seasoned Sprint executives who advised him to play it cool, Son soon clashed with regulators. With his deal hopes in tatters, he began slowly gutting the company.

Sprint executives had long seen the logic in a tie-up with T-Mobile, as a deal that would make it much less costly to build out both a 4G and a 5G-like network. But soon after SoftBank took over in 2013, Son’s management team at Sprint warned against moving too soon.

The Department of Justice and Federal Communications Commission in 2011 had just blocked AT&T from buying T-Mobile, citing fears that shrinking the number of national carriers to three from four could lead to price hikes for consumers.

“Our government affairs team made it very clear at the time it was the wrong move,” a former high-ranking Sprint executive with direct knowledge of the talks said. “It never occurred to us that he would not listen.”

Even Deutsche Telekom, T-Mobile’s parent, warned Son that it was the wrong time to try to get a merger through US regulators — despite telling SoftBank it was a willing seller, a source said.

Instead of heeding that advice, Sprint’s management recommended trying to win over FCC Chairman Tom Wheeler slowly over the course of a year — and then to seek regulatory approval to buy T-Mobile after Wheeler had warmed up to the idea, the source said.

That’s partly because Sprint’s staff had already witnessed Son interact with Wheeler’s predecessor, Julius Genachowski, when he was seeking FCC approval to buy Sprint — and they were worried.

“He had a caravan of five or six black SUVs that drove to the FCC meeting,” a source with direct knowledge of the meeting said. “They blow past the FCC security and park in Genachowski’s private driveway. They all pile out and go into the lobby.

“He was basically saying your rules are stupid,” the source said.

FCC officials were not pleased, the source added. But Genachowski, who is now a member of Sprint’s board, approved SoftBank’s purchase, partly on the strength of Son’s track record of buying and improving Japan’s No. 3 telecom player.

To cozy up to Genachowski’s successor, Sprint executives advised Son to attend a previously scheduled meeting with Wheeler later that year at Sprint’s Burlingame, Calif., labs. The idea was to show Wheeler what Sprint was doing with its advanced technology and convince him over time of the benefits of sharing that tech with T-Mobile, the source said.

But an impatient Son pushed ahead by scheduling a December 2013 meeting in DC with Wheeler in advance of the California powwow.

The DC meeting immediately got off on the wrong foot. That morning, a story had appeared in The Wall Street Journal about a proposed Sprint-T-Mobile deal. FCC officials took it as an attempt by Sprint to pressure the agency to OK the deal, a source told The Post at the time.

Wheeler told Son it would be difficult to OK a deal just yet. “He was telling Masa that they just did all this analysis about the [AT&T-T-Mobile] merger and unless things have changed in a year the analysis would be the same,” a source with knowledge of the meeting said. “Wheeler was trying to be helpful.”

The usually charming Son got defensive, sources said. “He began to argue in a way that wasn’t helpful,” the source briefed on the meeting said. “You could see Masa digging himself into a hole. It was not well played.”

Son refused to give up, and pressed his case for months to US senators. After realizing he would not move regulators, Sprint in August 2014 said it was ending its pursuit of T-Mobile.

The Wheeler visit to Sprint’s lab was canceled.

“Masa poisoned the well for a while,” the Sprint source said. “Had we waited a year, I believe we would have gotten the deal cleared,” the Sprint source said.

In arguing against a lawsuit by 13 states’ attorneys general to block T-Mobile’s acquisition of Sprint in December, Marcelo Claure, who ran Sprint from 2014 to 2018, used the company’s troubled state to argue in favor of the deal.

“The truth of the matter is we have the worst network quality scores in the industry,” Claure, who is now Sprint’s executive chair, told the judge.

“It was getting really, really hard to sell [service] because new customers went home, tested their phone, and then realized they had coverage gaps,” Claure said about Sprint’s problems going back to 2017. “So the amount of returns started skyrocketing, and therefore, employees no longer wanted to work for Sprint. And then the churn of employees or salespeople started to skyrocket and so that became a big problem.

“The quality of our network was four times worse than Verizon, half of T-Mobile and a little less than half of AT&T,” Claure added.

But back in 2013, when SoftBank was bidding for Sprint in a deal that included telecom company Clearwire, Son had been sitting on a gold mine, experts say.

“On the day that transaction closed, Masa was in an incredible position,” said New Street Research Analyst Jonathan Chaplin. Sprint owned much of the technology that would become the underpinning of 5G, and the Clearwire purchase gave it the spectrum it needed to offer its 5G network nationally.

Chaplin believes the stock might be worth $30 a share today had Son invested in Sprint. But rather than spend the $25 billion to $30 billion Claure at trial said it would have taken to compete, SoftBank pushed an unusual plan to build its network while saving on cell tower costs.

In 2014, Sprint kicked off a strategy to build a network without cell towers by getting permission from cities and towns to place their cell boxes on top of existing structures such as light poles.

“It was a 180-degree change,” the former Sprint executive said. “All of a sudden, we [in 2014] start on this tower and light thing.”

The plan had worked for SoftBank in Japan, and promised to let Sprint build its network “at a fraction of the cost of our competitors,” Claure told the judge at trial. “Unfortunately, I would say that it was a total failure,” Claure said.

A source involved in the failed rollout said SoftBank did not hire the public relations and marketing people it needed to win over town governments needed to OK cell boxes being attached to municipal property.

“It was insufficiently planned. They did not appreciate the politics,” the source involved in the failed rollout said.

In 2012, the year before SoftBank bought Sprint, the carrier spent $4.3 billion on capital expenditures, according to public filings.

After increasing spending by 25 percent to $5.4 billion in 2015, Sprint cut it in 2017 to $2 billion and in 2018 to $3.3 billion, levels below what they were before the SoftBank deal.

“In 2017, I believe we invested $1.3 billion on our network, while our competitors were investing six, seven, eight, nine ten billion dollars,” Claure said during his testimony.

Son initiated new merger talks in 2017 with T-Mobile, but they faltered amid bickering over which company would be the buyer. By this time, Sprint had lost its status as the nation’s third-largest network to T-Mobile’s colorful Chief Executive John Legere, who was upending the industry by freeing customers of onerous service contracts. In 2018, T-Mobile reached a deal to buy Sprint for $26 billion — dashing Son’s dreams of running the combined company.

A Manhattan federal judge on Tuesday knocked down the AGs’ bid to block the tie-up — sending Sprint’s stock up more than 70 percent.

Prior to the ruling, Sprint was trading at below $5 a share. Since SoftBank bought Sprint for $7.65 a share, T-Mobile has seen its shares soar more than 350 percent, while AT&T’s shares have risen about 65 percent, and Verizon’s 57 percent when adjusting for dividends and splits.

Sources say Son walked away from investing in Sprint because without T-Mobile he believed the company lacked the customer base needed to justify spending on the network.

In a 2015 interview with The Wall Street Journal, Son admitted he was ready to give up on Sprint in 2014 before claiming he chose to grow the network to protect Sprint’s customers.

After the deal failed, “I was thinking to myself, ‘I made one of the biggest mistakes in my life,’ which was the misjudgment of the US regulatory environment,” he said.

“We still have the customers, we still have employees, so I have to take care,” he said.

In the end, Son’s failure to invest in Sprint may have helped convince the judge to OK the merger — a hollow victory for the man who wanted to be the conqueror, not the conquered.

“Sprint’s trajectory over the last decade has been largely downward, as it has lost subscribers and been eclipsed by T-Mobile,” the judge said in arguing for a merger. “Due in part to several poor technological choices, Sprint’s network is poorer in quality than those of its competitors.”