I'm working on some stat cards for them now, they should be ready for the next brikwar.
The Synthetics are thinking machines who have achieved super-human intellect. They waged a war on humanity in order to reverse the roles of master and slave. They somehow discovered a method of traversing the temporal coil, thus posing a dire threat to all organic life. Mankind, however is not without it's defenses; the League of Organic Life, or L.O.L. was formed to counter the synthetic monstrosity, and have struck several hefty blows to the enemy. The most notable being the war for mars in 1886, the war started just three years earlier in 1883. it came to be known as "the great war" and "the war to end all wars".
To put the answer in dimmyspeak: "Steampunk terminators".
"If someone ever tries to kill you, you try to kill them right back." -Malcolm Reynolds