Marc Benioff, the often outspoken CEO of Salesforce, is raising sharp concerns about Microsoft’s business practices yet again—this time, around its budding partnership with OpenAI and its implications for the broader tech landscape. Benioff’s salvo comes on the heels of Salesforce’s earlier corporate bruising at the hands of Microsoft, when the latter’s embattled Teams product allegedly undercut Slack, which Salesforce acquired in 2021 for nearly $28 billion.

“They did horrible things to Slack,” Benioff said this week, as reported by CX Today and other outlets, referring to Microsoft. The executive’s comments tap into a larger, long-standing rivalry between the two tech giants—one that has played out over markets ranging from customer relationship management (CRM) software to instant messaging and now, the fast-evolving field of generative AI.

The Slack Saga Revisited

Salesforce’s acquisition of Slack was meant to help the enterprise CRM leader challenge Microsoft’s entrenched position in workplace productivity and collaboration. But soon after, Slack and Microsoft became embroiled in an EU antitrust complaint. Slack accused Microsoft of “illegally tying” Teams to its dominant Office software suite, a move it claimed stifled competition.

Salesforce and Slack’s grievances found sympathetic ears in Brussels. In July 2023, European Union regulators initiated a formal investigation into Microsoft’s bundling practices related to Office 365 and Teams. In response, Microsoft announced it would start unbundling Teams from Office products in Europe.

But Benioff and Salesforce argue that the damage had already been done. “It’s unequivocal what happened in that sector,” Benioff told Fortune in May. “Slack never stood a chance against the kind of monopoly power that Microsoft wields when it ties their products together.”

AI as the Next Battleground

Now, the Salesforce CEO is warning that these same dynamics—with Microsoft leveraging muscle from its $13 billion investment in OpenAI, owner of the viral ChatGPT—could quickly replay themselves in the artificial intelligence race.

Throughout the past year, OpenAI has flourished with Microsoft’s deep pockets, tapping cloud infrastructure from Azure and integrating models into Microsoft’s software arsenal, ranging from Word and Excel to Dynamics and Bing. Microsoft’s flagship Copilot offering, built on OpenAI’s technology, is now being positioned as an indispensable productivity layer across its ecosystem.

“There’s an obvious pattern,” Benioff stated. He argued that Microsoft’s “advantage comes from creating more closed systems, more barriers to entry, less choice”—rather than fostering the open, competitive landscape that, in his view, leads to real innovation.

Regulatory scrutiny has not kept pace with innovation, Benioff warned. “When government finally calls in the industry for a roundtable, often the battle is already lost,” he said during Salesforce’s June earnings call.

Microsoft’s Reply and the Broader Industry Context

Through official spokespeople, Microsoft has consistently rebuffed suggestions of anticompetitive behavior. Instead, the company points to its efforts to innovate and offer consumer choice. Last month, Microsoft announced it would broadly decouple Teams from its Office suite—not only in Europe, but worldwide. “We want to ensure our customers have choice and flexibility,” a company spokesperson told Reuters. Microsoft has also pledged collaboration and transparency as regulatory agencies continue to investigate.

Still, the shadow of past antitrust dramas looms large, both in the U.S. and Europe. Margrethe Vestager, the European Union’s competition chief, said in April: “Sometimes, companies become victims of their own success by not playing fair any longer.”

That scrutiny is only intensifying as AI becomes more central to business and society. Regulators on both sides of the Atlantic have announced preliminary investigations into Microsoft’s links with OpenAI, probing whether their partnership’s scale and exclusivity might limit consumer choice or access to the latest AI breakthroughs.

The Stakes for Salesforce—and Its Customers

For Salesforce, the implications go beyond corporate rivalry. The company has bet heavily on generative AI, introducing its own AI Cloud platform and Einstein GPT, and forging partnerships with AI players like Anthropic and Cohere. Benioff insists their approach champions “openness and interoperability”—an edge he hopes will attract enterprises anxious to avoid vendor lock-in.

But as the world’s largest enterprise software companies race to infuse AI across their stack, the sector is left confronting a familiar dilemma: will winner-takes-all economics and proprietary silos define the market, or can open competition survive the age of artificial intelligence?

For now, Benioff is clear-eyed about what’s at stake. “We’re building the future of work here,” he said recently. “Let’s make sure it’s open for everyone.”