Harry Turtledove 
Worldwar: Upsetting the Balance [EPUB ebook] 

Alien invaders from another star system have attacked the Earth, wreaking massive destruction and havoc with their terrifyingly advanced military technology. Humans are forced to unearth secret caches of terrible, once-forbidden weapons. Hitherto unthinkable tactics are brought into action against the alien enemy. Even as fuel shortages force people back to the horse and cart, the world’s scientists work desperately to create Earth’s first atom bombs – with horrific results. If the carnage and slaughter ever stop, will their be any pices to pick up? ’The wizard of If.’ Chicago Sun-Times ’The standard-bearer for alternate history.’ USA Today