A Day for Damnation — David Gerrold

Ravaged by plagues fatal to two-thirds of the population, Earth now faces the final violation... invasion of the giant, carnivorous Chtorr.

James Edward McCarthy was comandeered from hip college biology studies and sent off to fight alongside all able-bodied men and women. Now an officer in the Special Forces and a member of the secret Uncle Ira group, McCarthy is given the rare chance to contact the Chtorr. But when a helicopter crash leaves McCarthy and his companions stranded in territory claimed by the enemy, he must decide whether to kill the man-eating aliens ... or try to communicate with them.