alternatehistory.com

With a POD no later than 1800, have technology develop which will allow American forces (at least in the Confederacy) to develop Lovelace's spider machine by the time depicted in the movie. Assume the "laser guns" are flaming cannonballs.

Assume the movie takes place in 1875, midway through what would be Grant's tenure as president. The movie shows that he is president at the time.

Note that the Confederacy could win the Civil War in a situation like this (assuming the Union doesn't get the secret)...and wars are very good at spurring development of new weaponry.
Top