Microsoft has so far been waging its patent battle against Android indirectly, striking deals and going to court with makers of Android devices, not challenging Google itself. But it’s getting harder and harder for the two companies to avoid a direct fight over the issue. Here’s why.

Authorities in China this weekend gave Google the final regulatory clearance necessary to proceed with its $12.5 billion acquisition of Motorola Mobility. The deal is expected to be completed this week.

And late last week, the U.S. International Trade Commission gave Microsoft a victory in one of its patent battles with Motorola, issuing a final determination that Motorola won’t be able to import Android devices that infringe on Microsoft’s patent for “generating meeting requests and group scheduling from a mobile device.”

The patent relates to Microsoft’s widely used ActiveSync technology. It confirmed a preliminary determination by an administrative law judge. Unless it successfully appeals, Motorola will need to remove the technology from Android devices or strike a licensing deal with Microsoft.

Now, the decision on how to proceed will be up to Google, following the completion of its Motorola Mobility acquisition.

Google has made its feelings on the matter clear The company’s chief legal officer, David Drummond, spoke out in August against what he called “a hostile, organized campaign against Android by Microsoft, Oracle, Apple and other companies, waged through bogus patents.”

Microsoft deputy general counsel David Howard says in a statement, “Microsoft sued Motorola in the ITC only after Motorola chose to refuse Microsoft’s efforts to renew a patent license for well over a year. We’re pleased the full Commission agreed that Motorola has infringed Microsoft’s intellectual property, and we hope that now Motorola will be willing to join the vast majority of Android device makers selling phones in the US by taking a license to our patents.”

Motorola issued this statement, as quoted by PC Mag: “”Microsoft started its ITC investigation asserting 9 patents against Motorola Mobility. Although we are disappointed by the Commission’s ruling that certain Motorola Mobility products violated one patent, we look forward to reading the full opinion to understand its reasoning. Motorola Mobility will not experience any impact in the near term, as the Commission’s ruling is subject to a $0.33/per unit bond during the 60 day Presidential review period. We will explore all options including appeal.”

The ITC case is separate from another dispute between Microsoft and Motorola, in which the mobile phone maker says products including Xbox and Windows violate its patents. A federal judge in Seattle recently scolded the companies for “hubris” in that case, saying he was well aware that he was being “played as a pawn in a global industry-wide business negotiation.”

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