Google is planning to acquire Motorola Mobility for $12.5 billion. The reason's pretty simple: the latter's 17,000 patents (with another 7,000 pending) will provide Google with the legal ammunition it needs to defend Android against Apple, Microsoft and pretty much every other competitor in the mobility space.

"We recently explained how companies including Microsoft and Apple are banding together in anti-competitive patent attacks on Android," Google CEO Larry Page wrote in an Aug. 15 corporate blog posting. "Our acquisition of Motorola will increase competition by strengthening Google's patent portfolio, which will enable us to better protect Android from anti-competitive threats from Microsoft, Apple and other companies."

I reached out to Microsoft earlier this morning for comment. They had no comment, which wasn't very surprising. I'm sure lots of plans--and maybe a few chairs--are being hurled back and forth up in Redmond.

Microsoft's Android strategy had settled into something of a groove. Step 1: find a manufacturer producing Android devices. Step 2: attempt to enter a royalty agreement with said manufacturer. Step 3: if manufacturer balked, launch the lawsuit-waving attorneys.

Until now, that strategy worked. Microsoft had entered into royalty agreements with some big Android manufacturers and fired some lawsuits against a few others. A handful of analysts predicted that Android royalties could soon exceed Microsoft's take from Windows Phone sales.

But Google's Motorola acquisition changes that equation. For starters, Android now has more cover against Microsoft's patent infringement claims, potentially complicating any current and future lawsuits. Microsoft's particularly vicious battle against Motorola, I feel, is about to twist in a radical new direction.

Google's move could also blunt the impact of those 6,000 patents Microsoft and a consortium of partners (including Apple) paid $4.5 billion to acquire from Nortel earlier this year. In the wake of the purchase, Google made very public its concerns that those patents would be used to place Android in a legal choke hold--something Motorola's patents could help the search-engine giant escape.

But the acquisition also offers an opportunity for Microsoft, albeit one it will need to work hard to exploit. By swallowing Motorola, Google potentially alienates Samsung, HTC and other Android manufacturers. "On the one hand, Google and the OEMs share the common objective of maximizing Android activations," TBR analyst Kate Price wrote in an Aug. 15 research note. "On the other hand, Google is now a powerful, wealthy competitor, one that is impatient with the long lead times in the hardware business."

That could, she added, drive some of those manufacturers to give a second look at Windows Phone, which Microsoft has positioned as an Android alternative. "Microsoft will have to deliver a truly competitive consumer mobile operating system to take advantage of this opportunity," she wrote. "To leverage this acquisition, Google will have to work closely with OEMs to reassure them that they are still a trusted partner, and will have to resist the temptation to work more closely with Motorola Mobility to shorten lead times and deliver innovation faster."

If Google fails to reassure those partners, and if Microsoft plays its cards right, then we could see Windows Phone market share grow over the next several quarters. That could prove difficult, however, given the smartphone platform's slow but steady market-share slide, something it could take considerable effort to reverse even with more manufacturers aboard.

In the short term, though, Microsoft is probably more concerned with how Google's move will affect things in the courtroom. "This will provide more balance and a better defensive position for Android, which was getting killed in court," Rob Enderle, principal analyst of the Enderle Group, wrote in an Aug. 15 email to me.

Remember when late August used to be a slow time of year?