The big imperial counter attack that counts as an imperial victory somehow even though it did exactly what i described: iom rolls in and stomps over initial tau resistance (tau practice elastic doctrine so iom wasted time on lightly defended early warning garrisons) and then tau just outsmart the iom as the humans do silly things like sending waves of guardsmen unsupported straight into tau gunlines.
This is when iom lost titans to mantas because said titans were unsupported, where all space marines who werent ultras got wrecked including the raven guard and where kroot beat catachans at jungle warfare and get eaten.
But the iom was totally going to win! Even though they were described as being on the defensive with the biggest imperial battlefield wins coming from sm buying time for imperials to evacuate, wow such win.
Then the tyranids arrived so the imperial armada had to leave to fight on the other side of the galaxy, providing a nice cop out excuse.
Then the tau immediately ignored all signed promises and agreements like they always do and went back in and reclaimed all their lost turf whilst dealing with the stranded imperials left behind, before expanding once more, thus ending with more territory then they started the war with.
Later on, the ultramarines by themselves would reclaim some of the lost territory in the zeist campaign or whatever it was called, whether the ultras accomplishing more alone then the grand invading imperial armada is the ultras being plot armored or proof of how incompetent the imperial leadership was during damocles i’ll let others decide.