In the Ender's game movie, Ender is told near the end that that
The Formics are building up military power, and that's why they can't wait to invade.
Is this reason for the I.F. rush backed up by the book? I seem to remember that the rush was because the ships were about to get to the Formics.
To clarify my question, why didn't the I.F. slow down their ships and wait a few months so that Ender could get fully trained?