As many people have noted in their answers, the battle tactics of an era depends a great deal on the technology available. It is difficult to imagine that any sort of space empire will not be able to field equipment comparable with what a modern 21rst century soldier is equipped with, except perhaps much lighter, more reliable and more lethal.
Little Green Men. Because it is a space fantasy, right?
So the issue needs to be how to eliminate all these technological advantages, and in such a way that it does not negate everything else. In Joe Haldeman's novel "Forever War", a soldier in an interstellar war discovers on returning from a mission that due to time dilation, centuries of R&D have happened in his absence and a "Stasis Field" has been developed, which slows the movement of every sort of particle to some arbitrarily slow speed. Electronics no longer work, engines and computers seize up and even biological processes stop. In the novel, covering your spacesuit or powered battle armour in a special protective coating protects you from the effects of the Stasis field, but if someone wants to come in and fight (assuming they are protected as well), then only edged weapons like swords and spears will work....
So the Stasis field is a bit extreme for what you want to do, but there are currently understood technologies which, if developed to extremes, could have the same practical effects on the battlefield. Highly focused EMP weapons could be used to burn out electronics on all but the most heavily shielded devices. Highly advanced cyber warfare weapons would reduce targeting computers, software radios and other computerized devices into junk (or untrustworthy devices with trojan horses and logic bombs implanted that could go off at critical times). The orbiting starships could ensure that no air or enemy spacecraft of lesser size and power than the starship itself could operate over the battlespace.This also applies to military vehicles large enough to be observed from orbit.
Now soldiers would be limited to communicating by voice, and advanced weapons systems of all sorts would be ineffective. To reduce weaponry to muskets and bayonets, there would need to be some sort of way of disabling modern explosives. Perhaps the battlespace was seeded with some form of nanobot plague designed to "eat" or neutralize modern explosives (perhaps initially a means of neutralizing IED's), but now gone rogue and mutated. However, traditional black powder isn't to its taste, so black powder weapons have made a comeback.
There are a few things which even this scenario isn't going to change. Modern machining means that the muskets can be built to exacting tolerances and rifled, meaning they will be deadly accurate to much longer ranges. The idea of cartridges isn't going away, they are far too convenient as well. So you would actually be looking not at the Napoleonic wars, but the period between the American Civil War and the introduction of automatic weapons in the 1880's 1890's (not volley fire or hand crank weapons like the Gatling or Nordenfelt gun, but true machine guns). So there will be a much greater use of open order and dispersed tactics in order to prevent forces from being gunned down.
10 barrel Nordenfelt gun
Artillery will see the same dynamic, modern machining means there is no need for muzzle loading cannon and rifled breach loaders are far more accurate and dangerous. Firing tables printed in books or mechanical calculators will also be used to rapidly calculate ranges and make artillery a very potent arm.
Canon de 75 modèle 1897. That is just going to wreak your entire day
So assuming the technological means to negate communications, electronic and computerized weapons and devices, and some means of negating modern explosives (and indeed many chemical weapons), then you will be thrown back into the 1800's. However, because it is still possible to manufacture weapons using modern materials and techniques, you will be in the mid to late 1800's in terms of technology, and your solders will be fighting in the manner of the American Civl War or the Franco-Prussian war.