Cain recalled the battles they had fought along the way. Though the expeditionary fleet had a major mission, it would still answer distress signals from nearby sectors and go clear a few dungeons or whatever.
So when an expeditionary fleet turned back midway, it was very likely because the dungeon ahead had been too hard to clear.
The Imperium had launched several expeditions against the T'au Empire, hoping to crush that little empire outright. But every time, sudden incidents within its own borders diverted troops away, until the massive expeditionary fleet was reduced to only a scattered handful.
In terms of total strength, the Imperium could crush any enemy. The problem was that it faced far too many enemies, and out of every ten parts of its strength, it could often only bring one to bear.
Warp travel was terrible and unreliable, entirely dependent on luck.
Once a fleet entered the Warp's currents, it was blind. In a place where space and time had lost all meaning, they could only rely on the Navigator's judgment.
For example, you might enter the Warp one moment, and the next, the Navigator would say, "All right, tear open a breach here and we're there."
Or perhaps you drifted through the Warp for so long that it felt like the Navigator was about to die of old age, yet when you tore open the breach, you still arrived at your destination.
You thought it had been a short time, but a hundred years had passed. You thought it had been an eternity, but only a few seconds had gone by.
Thus, the Imperium had developed the strange condition of overwhelming everything in general, while suffering an extreme disadvantage in specific situations.
For the Imperium, losing a fight simply meant there were not enough troops.