Sci Fi book where the earth defending space pilots fight an invading alien fleet through telepresence or so they think. Turns out later the invading force is humans who left system years before. Read before 2005. Spoilers ahead.

[Adrian]

Read pre 2005. Could be young adult but could also be full adult novel as id read most sci fi books in town library by that stage of space opera variety.

[Steve]

Sounds like Ender's Game by Orson Scott Card.

https://www.google.com/books/edition/...

[Kris]

Google Books preview of Ender's Game by Orson Scott Card - Steve's suggestion:

https://books.google.com/books?id=Ojq... There are different cover images.

Wikipedia's plot summary (with Spoilers):
https://en.wikipedia.org/wiki/Ender%2...

[Bargle]

Steve wrote: "Sounds like Ender's Game by Orson Scott Card.

https://www.google.com/books/edition/..."

No, it doesn't. In Ender's Game the invading fleet is aliens.