In a move equal parts strategic and defensive, Microsoft will acquire, for about $6 billion in cash, aQuantive, a company that specializes in online and interactive advertising.

The move follows the $3.1 billion purchase of another large online/interactive ad firm, DoubleClick, by Google, which is developing into something of an archrival to Microsoft.

Similarly, Time Warner’s AOL unit recently bought a controlling interest in German online ad company Adtech and also purchased Third Screen Media, an ad agency that targets mobile handsets. Meanwhile, WPP, a large advertising and media company, decided to buy digital marketing company 24/7 Real Media for $649 million.

AT&T didn’t buy anybody, but still managed to insert itself in the new-media ad frenzy by announcing it expects that in three years it will be making a combined $1 billion a year from advertising on its mobile phones and on its U-verse triple play service.

Microsoft has long touted the value of online advertising, but the rash of acquisitions compelled the company to try to bring some of that capability in-house. Microsoft’s spin will be that it will be able to provide the development tools for online and interactive advertising.

To wit, according to Microsoft CEO Steve Ballmer: “The advertising industry is evolving and growing at an incredible pace, moving increasingly toward online and IP-served platforms, which dramatically increases the importance of software for this industry.”

aQuantive is the parent of Avenue A | Razorfish, an ad agency that specializes in interactivity; Atlas, a digital marketing company; and DRIVEpm, a reseller of online ad space.