The first day of a weeklong listening to that might decide the result of Microsoft’s $70 billion acquisition of the online game big Activision Blizzard opened Thursday with a promise from Microsoft: If a federal decide grants an injunction that will delay the deal’s closing, Microsoft may abandon the deal altogether.

“This is going to decide whether the deal goes forward,” mentioned Beth Wilkinson, Microsoft’s lead lawyer. She added {that a} loss may drive the corporate right into a “three-year administrative nightmare” that will sink the transaction, which it hopes to shut by July 18.

That set the stakes for the federal court docket listening to in San Francisco, the place the Federal Trade Commission started laying out its case that Microsoft buying Activision — and its well-liked video games like Call of Duty — could be devastating for the online game business.

The F.T.C. is asking Judge Jacqueline Scott Corley for a preliminary injunction, which might bar Microsoft from finishing the deal earlier than the F.T.C. has had the possibility to argue the case in its inner court docket.

The conflict is broadly seen as a check of whether or not latest efforts to extra aggressively curb the ability of tech giants around the globe will succeed. Lina Khan, the chair of the F.T.C., has argued that large tech corporations have huge affect over on-line commerce and communication, permitting them to interact in anticompetitive practices that hurt customers.

“If this deal is completed, the combined company will have and is likely to have the ability and incentive to harm competition in various markets related to consoles, subscription services and cloud,” mentioned James Weingarten, the F.T.C.’s lead lawyer, in court docket on Thursday.

Mr. Weingarten additionally mentioned Microsoft may make Activision’s video games unique to its Xbox console or degrade their high quality on different platforms to make the Xbox extra interesting to players. He pointed to Microsoft’s previous $7.5 billion buy of ZeniMax Media and its slate of sport studios in 2020, after which Microsoft made a few of these video games unique to Xbox.

The F.T.C. can be anticipated to argue that absorbing Activision’s video games into Microsoft’s portfolio would give it an unfair edge within the nascent marketplace for cloud gaming.

Microsoft responded that the Activision deal could be good for customers, increasing their capability to play Activision’s video games by low-cost choices like Microsoft’s Xbox Game Pass, a subscription service, or by the platforms of different corporations like Nintendo and Nvidia, with whom it has partnered.

Ms. Wilkinson additionally argued that it will be nonsensical for Microsoft to take away Call of Duty and different titles from different platforms, reminiscent of Sony’s PlayStation, as a result of the corporate would lose out on an enormous chunk of the sport’s income. She mentioned Sony had turn out to be the “complainer-in-chief” within the case, and confirmed an e-mail from Sony’s chief government, Jim Ryan, suggesting that he didn’t actually consider Microsoft would withhold Call of Duty.

The F.T.C. has in one other lawsuit accused Meta, Facebook’s dad or mum firm, of slicing off nascent opponents when it purchased Instagram and WhatsApp. On Wednesday, it sued Amazon over allegations that the corporate tricked customers into signing up for its Prime subscription service. But the F.T.C. has had setbacks: Earlier this yr, its problem to Meta’s buy of a digital actuality start-up fell apart after a decide declined to cease the deal from closing.

The F.T.C. initially challenged Microsoft’s bid for Activision utilizing an in-house court docket. But that court docket doesn’t have the authorized authority to cease the deal. The F.T.C. requested the U.S. District Court for the Northern District of California to step in earlier this month, saying it feared Microsoft would attempt to full the deal regardless of the authorized challenges.

The listening to in Judge Corley’s courtroom may very well be a decisive check for the F.T.C. If Microsoft wins, it will sign that there are weaknesses within the F.T.C. case and will trigger the company to drop its problem to the deal. But a win for the F.T.C. may very well be an indication that its broader problem has legs, and will put new strain on Microsoft and Activision to rethink the multibillion company marriage.

Sony, whose PlayStation console competes in opposition to Microsoft’s Xbox, has been a vocal critic of the deal. Sony argues that PlayStation players may lose entry to Call of Duty — a large franchise that has earned greater than $30 billion in lifetime income — if Microsoft determined to make the sport unique to Xbox. Microsoft has denied that it will achieve this.

Though most governments around the globe, together with the European Union, have authorized the acquisition, Microsoft was dealt a setback in April when a British regulatory authority blocked it. That resolution is underneath attraction.

The high-profile checklist of witnesses anticipated to testify earlier than Judge Corley over the following week consists of Satya Nadella, the chief government of Microsoft; Amy Hood, the corporate’s chief monetary officer; Bobby Kotick, the chief government of Activision; and Phil Spencer, the chief government of Microsoft’s Xbox unit. Mr. Ryan, the chief government of Sony, will seem through a prerecorded video deposition.