Every society selects for something: generally it looks as if the direction of selection pressure is more or less an accident. Although nations and empires in the past could have decided to select men for bravery or intelligence, there’s not much sign that anyone actually did this. I mean, they would have known how, if they’d wanted to, just as they knew how to select for destriers, coursers, and palfreys. It was still possible to know such things in the Middle Ages, because Harvard did not yet exist.
A rising empire needs quality human capital, which implies that at minimum that budding imperial society must not have been strongly dysgenic. At least not in the beginning. But winning changes many things, possibly including selective pressures. Imagine an empire with substantial urbanization, one in which talented guys routinely ended up living in cities – cities that were demographic sinks. That might change things. Or try to imagine an empire in which survival challenges were greatly reduced, at least for elites, so that people had nothing to keep their minds off their minds and up worshiping Magna Mater. Imagine that an empire conquered a rival with interesting local pathogens and brought some of them home. Or one that used up a lot of its manpower conquering less-talented subjects and then importing masses of those losers into the imperial heartland.
If any of those scenarios occurred, they might eventually result in imperial decline – decline due to decreased biological capital.
Right now this is speculation. If we knew enough about the GWAS hits for intelligence, and had enough ancient DNA, we might be able to observe that rise and fall, just as we see dysgenic trends in contemporary populations. But that won’t happen for a long time. Say, a year.