"Usagi Yojimbo: Senso" (originally given the working title "War of the Worlds") is a Usagi Yojimbo storyline. It is loosely based upon the H.G. Wells novel of the same name. Stan Sakai has stated that this story will not appear before 2010 due to the fact that "it makes more sense to concentrate on regular Usagi" since 2009 is the 25th anniversary of Usagi Yojimbo. 
From the UY Dojo:
"This story is about a small expeditionary party of three ships that the Martians sent to earth in 1628--more than two centuries before their big invasion. Usagi has joined the Geishu clan by this time, and Lord Noriyuki has grown into a fine, capable lord. Lord Hikiji has made his move to overthrow the shogunate, and the county is driven into another civil war. It is during the great battle between the Geishu samurai and Hikiji's forces that the first of the Martian ships crashes into the middle of the field. Stan has some nice images in mind--a huge tripod behind a Japanese castle, Edo being razed, armored samurai attacking a martian ship, and the Neko Ninja (again led by Chizu) infiltrating a tripod. To readers who can't grasp such a wild twist in the pages of UY, Stan says he taken all doubts and concerns into consideration, and the story will fit into Usagi continuity and Japanese history."