Created by Blackwave, Swarm on the Somme deals with an attack on Earth in early 1915 by the Grex, a vicious extraterrestrial race of insects, or 'Roaches', as they are soon nicknamed. Landing initially in northern France, right between the frontlines of the Entente and Central Powers, in Siberia, and in the Australian Outback, they soon spread into North America and Japan, with the situation looking increasingly desperate for humanity.
Still in progress, it can be read here. If you want to read it as a compilation without comments, go here.
A thread compiling various artwork done by readers of the timeline can also be viewed here.
Another thread by Dan Reilly the Great showcases some of the 3D fan art he created based on the timeline.
Swarm on the Somme has won the 2010 Turtledove Award for "Best New ASB Alien Contact Timeline", and then the "Best Overall ASB Timeline" award.