I must have read this book in my teens but can't find any reference on the web.
If I remember correctly, it was about mankind being at war with some alien race. Waging bloody war etc.
At some point the main character discovers that these are not aliens but humans. Humanity fights itself to keep the economy running.
At least that is what I remember.
Are you sure this isn't The 5th Wave by Rick Yancey? It's from 2013, but it otherwise matches your description exactly.
The novel describes an invasion of Earth by aliens known as "the Others", who attack Earth with, in order: a planet-wide EMP; kinetic bombardment; an engineered avian flu pandemic; and a group of Others disguised as humans who infiltrate and kill the survivors.
The novel follows a teenage girl named Cassiopeia (aka "Cassie") as she survives the first four waves, and is then taken to a military camp with a group of other survivors to train as resistance fighters and combat the impending "Fifth Wave" assault of possessed humans, using implants that will show them which humans are actually Others.
The twist is that they're not fighting the Fifth Wave - they are the Fifth Wave. The military camp is actually being run by the disguised Others from the Fourth Wave, with the aim of tricking the survivors into wiping each other out, and the implants flag up anyone not wearing an implant as an Other.
(Disclaimer: I haven't read the book, but I did watch the 2016 film adaptation.)
