Microsoft and Google are resuming open hostilities, following a five-year cease-fire.

Microsoft and Google had an agreement, beginning five years ago, to stop suing each other and work out their differences behind closed doors. During that period, the two companies worked closely together on some projects, not the least of which is Microsoft adopting Chromium as the rendering engine for its Edge web browser. The company has become one of the biggest contributors to the open source rendering engine, with many of Microsoft’s improvements making their way into Google’s Chrome, also based on Chromium.

Despite the benefits of the two companies’ non-aggression pact, that pact has ended, driven largely by Microsoft’s growing frustration with Google’s approach to its ad business and the debate around paying content creators.

According to Bloomberg, tensions started building over Google’s reluctance to pay news publishers for content. In contrast, Microsoft has been a proponent of reimbursing news publishers and content creators. The breaking point, however, was Microsoft’s frustration about marketers not having equal access to search engines when using Google’s ad campaign tools. 

“If you want to advertise, if you want to sell advertising or buy advertising on the internet, you have to use Google’s tools, and when they make their tools in a manner that fails to interoperate easily with others, it impacts everybody,” said Microsoft President and Chief Legal Officer Brad Smith, in April, in a Bloomberg television interview. “We raised the concerns with them and they just turned a deaf ear.”

The two companies already compete on vast array of fronts, not the least of which is the cloud. Now that the gloves have come off, it will be interesting to see just how far the hostilities go.