If one civilization pursues genetic engineering to create superhumans with superior intellect and everything else, how can other civilizations maintain a balance of power, apart from doing the same.
Focusing on Other Things
One possibility is other nations put their effort towards better materials, better tanks and engineering, better economics and the like. That, even with the more intelligent humans, the genetic engineering culture simply hasn't had time to catch up.
If the genetic engineers had a much weaker nation, then even with their advantages they may still be behind for a long time.
Further on the previous point, if the richer nations started hiring some of the genetic engineering nation's genetically engineered talent, that could allow the other nations to parasitically enjoy some of that progress, and keep things even for longer.
If the genetic engineering nation isn't careful, everyone will start to see its strength, and worry about it, and make alliances with one another to counteract its strength.
It's possible the genetic engineering does not pan out as well as hoped, or at least not as often. If for every genius you get someone suffering from horrible cancer, or only a small percentage of those engineered possess remarkable positive traits, then the difference will be noticeable but not game changing for quite some time.
Or, if they make a nation of geniuses, but they become vulnerable to certain diseases or the people become more independent and less interested in war, various possible factors could lead to issues with pursuing a war.
If “everything else” includes high moral character and altruism, this is actually better than having two “primitive” neighbors fighting over resources and stupid fueds. The superhumans will ensure that their more-natural neighbors are not exploited or cheated. Those are such unevolved characteristics, after all.
The genetic enhancements are being used to provide some sort of advantage to the people being modified, so rivals who are adverse or unable to match genetic engineering will need to provide similar enhancements some other way.
The most current method is to create and maintain a high trust society where people have lots of linkages and access to resources. These societies tend to be wealthier and capable of more rapid advancement than their rivals, and across a much wider spectrum of social, economic and other fields. For example, Russia currently has a GDP similar to Italy. It is an autocracy or oligarchy, and much of the resource base is consumed in military adventurism or siphoned off by the ruling class. Russia does not make any domestic coffee makers, despite the relative simplicity of these consumer goods because of the economic distortions created by their political class. Italy, on the other hand, produces,coffee makers, very high end luxury goods like sports cars, art and fashion, yet also has the ability to produce regular cars and a plethora of goods, foodstuffs, computer and other services (and service industries) with about the same sized economy. Even the average lifespan is longer in Italy. Genetically enhancing Russians isn't going to materially change that situation.
The next step would be to match genetic modifications with mechanical enhancements. If changing genes makes a person stronger, individuals who are not genetically enhanced can be given devices to increase their strength. Exoskeletons are a common idea, but realistically, no amount of individual enhancement could match the digging ability of a backhoe or the lifting capability of a crane. Networked computers with access to large databases will certainly be able to provide more information to a person than tweaking a brain (although with cognitive enhancements, the modified person might think differently).
From there we can go to the use of drugs to modify metabolisms, reduce or eliminate the need for sleep, provide surges of strength and other effects.
Finally, we can start replacing parts of the body with cybernetic enhancements, and create cyborgs with potentially far greater performance than any biologically enhanced humans.
So long as there is a compelling need to "enhance" people to perform various tasks, clever scientists and engineers will develop products and services which allow humans to perform more work and work in a broader range of environments. Genetic engineering may well be part of that effort, but since the genetic engineers are ultimately dealing with the limits of flesh and blood, there will be a fairly hard upper boundary to what genetically engineered humans could do.
One thing that can't be "engineered" is desirability. Look at the differences between the planned communities of the 1950s and 2010s. What looks beautiful and futuristic and advanced today will probably look as lame to the people of the future as the new Library of Congress building looks to us today.
A super-society comprised of super-citizens may be so awful in reality that none of the super-citizens bears children, or something. There was a reason the Greek gods punished human hubris.