He paused, and there was a deep silence.

"That's what we're up against: a race which offers no quarter because it knows it will receive none. I don't say it's a situation which can never be changed, but clearly it's one we cannot hope will change in time to save us.

"On another level, there are things about the Aku'Ultan we don't pretend to understand.

"First, there are no female Achuultani." Several people looked at him in open disbelief, and he shrugged. "It sounds bizarre, but so far as we can tell, there isn't even a feminine gender in their language, which is all the more baffling in light of the fact that our prisoner is a fully functional male. Not a hermaphrodite, but a male. Fleet Captain Cohanna suggests this may indicate they reproduce by artificial means, which might explain why we see so little variation among them and, perhaps, their apparent lack of evolutionary change. It does not explain why any race, especially one as driven to survive as this one, should make the extraordinary decision to abandon all possibility of natural procreation. We asked Brashieel about this and got a totally baffled response. He simply didn't understand the question. It hadn't even occurred to him that we have two sexes, and he has no idea at all what that means to our psychology or our civilization.

"Second, the Nest is an extremely rigid, caste-oriented society dominated by the High Lords of the Nest and headed by the Nest Lord, the highest of the high, absolute ruler of all Achuultani. Exactly how High Lords and Nest Lords are chosen was none of Brashieel's business. As nearly as we can tell, he was never even curious. It was simply the way things were.

"Third, the Aku'Ultan inhabit relatively few worlds; most of them are always away aboard the fleets of their 'Great Visits,' sweeping the galaxy for 'nest-killers' and destroying them. The few planets they inhabit seem to be much further away than the Imperium ever suspected, which is probably why they were never found, and the Achuultani appear to be migratory, abandoning star systems as they deplete them to construct their warships. We don't know exactly where they are; that information wasn't in Vindicator's computers or, if it was, was destroyed before we took them. From what we've been able to determine, however, they appear to be moving to the galactic east. This would mean they're constantly moving away from us, which may also help to explain the irregular frequency of their incursions in our direction.

"Fourth, the Nest's social and military actions follow patterns which, as far as we can tell, have never altered in their racial memory. Frankly, this is the most hopeful point we've discovered. We now know how their 'great visits' work and how to derail the process for quite some time."

"We do?" Gerald Hatcher scratched his nose thoughtfully. "And just how do we do that, Hector?"

"We stop this incursion," MacMahan said simply. There was a mutter of uneasy laughter, and he smiled very slightly. "No, I mean it.

"The Achuultani possess no means of interstellar FTL communication other than by ship. How they could've been around this long and not developed one is beyond me, but they haven't, which means that once a 'great visit' is launched, they don't expect to hear anything from it until it gets back."

"That's good news, anyway," Hatcher agreed.

"Yes, it is. Especially in light of some of their other limitations. Their best n-space speed is twenty-eight percent of light-speed, and they use only the lower, slower hyper bands—again, we don't understand why, but let's be grateful—which limits their best supralight speed to forty-eight lights; seven percent of what Dahak can turn out, six percent of what the Guard can turn out under Enchanach Drive, and two percent of what it can turn out in hyper. That means they take a long time to complete an incursion. Of course, unlike Enchanach Drive, there's a time dilation effect in hyper, and the lower your band, the greater the dilation, which means their voyages take a lot less time subjectively, but Brashieel's ship had already come something like fourteen thousand light-years to reach Sol. So if the incursion sent a courier home tomorrow, he'd take just under three centuries to get there. Which means, ladies and gentlemen, that if we stop them, we've got almost six hundred years before a new 'great visit' can get back here. And that we know where to go looking for them in the meantime."

A soft growl came from the assembled officers as they visualized what they could do with five or six centuries to work with.

"I'm glad to hear that, Hector," Hatcher said carefully, "but it leaves us with the little matter of three million or so ships coming at us right now."

"True," Colin said, waving MacMahan back down. "But we've learned a little—less than we'd like, but a little—about their strategic doctrine.

"First, we have a bit more time than we'd thought. The incursion is divided into three major groups: two main formations and a host of sub-formations of scouts which do most of the killing. The larger formations are mainly to back up the scout forces, each of which operates on a different axis of advance. Aside from the one which already hit us, they're unlikely to hit anything but dead planets as far as we're concerned, and a half-dozen crewed Asgerds could deal with any of them. If we can stop the main incursion, we'll have plenty of time to hunt them down and pick them off.

"The real bad news is coming at us in two parts. The first—what I think of as the 'vanguard'—is about one and a quarter million ships, advancing fairly slowly from rendezvous to rendezvous in n-space to permit scouts to send back couriers to report. We may assume one's already been dispatched from Sol, but it can't pass its message until the vanguard drops out of hyper at the rendezvous, thirty-six Achuultani light-years from Sol. Given the difference in length between our years and theirs, that's about forty-six-point-eight of our light-years. The vanguard won't reach their rendezvous for another three months; we can be there in about three and a half weeks with Dahak, and a hell of a lot less than that for the Guard in hyper.

"And take on a million ships when you get there?" Hatcher said.

"Tough odds, but I've got a mousetrap planned that should take them out. Unfortunately, it'll only work once.

"That's our problem. Even if we zap the vanguard, that still leaves what I think of as the 'main body': almost as numerous and with some really big mothers, under their supreme commander, a Great Lord Tharno.

"Now, the vanguard and main body actually keep changing relative position—they 'leapfrog' as they advance—and their rendezvous are much more tightly spaced than the scouts' are. Again, this is to allow for communication; the scouts can't pass messages laterally, and they only send one back to the closest main fleet rendezvous if they hit trouble, but the leading main formation sends couriers back to the trailing formation at each stop. If there's really bad news, the lead force calls the trailer forward to link up, but only after investigating to be sure they need help, since it plays hell with their schedules. In any case, however, at least one courier is always sent back and there's a minimum interval of about five months before the trailer can come up. With me so far?"

There were nods, and he smiled grimly.

"All right, that's our major strategic advantage: their coordination stinks. Because they use hyper drives, their ships have to stay in hyper once they go into it until they reach their destination. And because their maximum fold-space com range is barely a light-year, the rear components of their fleet always jump to the origin point of the last message from the lead formation. Even in emergencies, the follow-on echelon has to jump to to almost exactly the same point, assuming they mean to coordinate with the leaders, because with their miserable communications they can't find each other if they don't."