Another patent war is brewing, and for once, neither Apple (News - Alert) nor Samsung is involved. This time the combatants are Facebook and Yahoo, and Facebook (News - Alert) is throwing first punch.

While Yahoo recently came after Facebook over the alleged infringement of a dozen patents, Facebook responds that Yahoo's hands aren't quite clean either, as Yahoo infrings on 10 of Facebook's own patents.

Most of the patents in question revolve around Yahoo's optimization engine, and large parts of Flickr as well. The suit itself also claims infringement via Yahoo's home page, and advertisements spread throughout much of Yahoo's sub-services including Yahoo Finance, Yahoo Sports, Yahoo Games and several others.

For its part, Yahoo reportedly issued a statement implying that while they only just received Facebook's answer to Yahoo's suit against Facebook and the details their countersuit, Yahoo believes they are both “without merit” and “a cynical attempt to distract from the weakness of its defense.”

The timing of the legal battle isn't particularly surprising either, as Facebook is pending an initial public offering which will give Yahoo a shot at either a hefty award or some kind of settlement. Using timing to its fullest is nothing new for Yahoo, having sued Google (News - Alert) over infringement back in 2004, a move which left Yahoo owning a substantial portion of the search titan.

But looking at Facebook's countersuit, the terms at stake become quite clear. Yahoo advertising, which is a large parcel of what Facebook claims Yahoo is infringing, represented over $4 billion in 2011, and the introduction of the CORE (Content Optimization and Relevance Engine) system has yielded a 300-percent increase in click-through rate, according to Facebook's suit.

Both companies have a lot to lose here, and it will take a court of law to ultimately determine just how valid each company's grievances truly are. With Facebook in the midst of an IPO, and Yahoo's primary income driver the target of a lawsuit, it's not hard to wonder if a settlement will be in the offing, or if either side will fight until the bitter end.