But things only got that desperate -- to the point Colin had to do the nova trick and Dahak had to sacrifice himself -- because Colin was much more limited than those ships "should" have been (had they actually been supported by the 4thImp).
His ships' missiles massively outranged the Achuultani ones.
If he'd had the full several years' warning of the sensor arrays were designed to provide (rather than having to do the whole Anu thing, then go pick up the starships at Birhat... and Dahak got the messages even before he kidnapped Colin), and had had enough missiles (I'm pretty sure they were drastically short on missiles), he could have gone out and blasted them with missiles from beyond their range, then retreated and done it again and again till their whole fleet was destroyed.
But by the time he got the ships he was down to the wire and didn't have either the time or space (before they reached Earth) to repeatedly fall back that way, nor the missiles to destroy them all. So he had to close to energy range, which is why his ships got shot up so horribly.
And I don't think they were that
ignorant of the Achuultani. Dahak points out that the remains of that shattered planet (Sheskar maybe? the old Fleet base one) are inconsistent with the effects of Achuultani weapons. And it's commented that the scouting force was twice as big as they expected. Both of those statements are before they capture the Achuultani computers, so those would have to be based on pre-existing 4thImp knowledge of the Achuultani.
And whatever the 4thImp did or didn't know about the Achuultani, the 4thEmp
didn't believe they existed anymore. So what were they -- as a single government which didn't know about/believe in any other political entities in the universe -- doing with the planetoids? 1,000,000 planet-destroying ships seems way, way, way too much to deal with revolutions and terrorists and such.
Why did the 4thEmp have a million planetoids? Was it a matter of maintaining 4thImp military buildup levels, or did they build even MORE after Herdan?
I mean, Colin beat a double-sized Great Visit with less than a hundred undermanned ones, so a million seems absolute overkill for anything, ever.
You're forgetting the fact that if Dahak hadn't sacrificed himself to destroy the master control computer in charge of the remaining Achuultani fleet, Colin and his few remaining ships couldnt have done anything more than flee to Earth and evacuate as many civilians as possible. At the time that Dahak killed the other computer, the Achuultani still had more than enough ships left to finish the job.
Also, dont forget that Colin and company had surprise on their side the entire time. They were able to lock the Achuultani out of hyper space which seriously screwed with their ability to move and fight.
No, without the surprise factor and the trick Colin used to destroy half of the great visit in a single blow. There is no way they would have come anywhere close to winning. In a stand up fight, without the element of surprise, Colin would have lost and done so in a rather spectacular way.
As for why they had so many ships. You seem to have forgotten that they knew the Achuultani would return at some point. That information wouldn't simply disappear. Their military was built to deal with that specific threat. They also knew that the time between Achuultani visits was measured in tens of thousands of years. They also knew that despite their best efforts in the past, it had been shear dumb luck that left one of their worlds intact each time the Achuultani came to kill them.
Based on that information, there is no way, they would have forgotten why they needed the fleet they had built. They expected to win this time, but they also expected to pay for the win with heavy losses of their own.
They also didn't know much about the Achuultani. They were a backwater colony world with minimal contact with the main Empire. After they regained space travel, they had to reconstruct what had happened from fragments. All they knew was that an absolutely enormous number of ships with unknown capabilities from an unknown alien power had destroyed every single other planet in what they had originally believed was a strong empire. Lacking detailed information about the enemy, it's ships and their capabilities; they would have had no way of knowing if even a million planetoids would be enough.