I'm just brainstorming here, but if you were to fight a foreign army, and a foreign population, why not deploy a virus which targets the germline stemcells, thus making the entire populace sterile.
Or, better yet, just have the virus target XY sperm, so that the populace will be made up nearly completely of women.
If we created a sterlizing biological weapon, and used it against foreign states, it would effectively eliminate the capacity of a high body count war. If they lose most of their young men in fighting, more young men won't be created to replace them.
If their sperm adapts, so will the virus.
This would have a much weaker political impact than using a virus to wipe out an entire nation. Furthermore, we have been violating most of our treaties anyways, so it wouldn't be a big stretch.
If the U.S/Western/Industrialized Country men got this virus, they could simply use anti-virals.
So it would make it harder to reproduce in the undeveloped world, but easier to reproduce in the industrialized world.
It would make it where having male offspring (which are the most aggressive, and likely to go to war) wouldn't populate also among groups that are less economically privileged, so that the groups that are most likely to revolt, and overthrow the government/become terrorist/criminals/thugs would be produced in smaller numbers.
This would also stem a large amount of the illegal immigrant problem, because the virus wouldn't be contained to any one nation. So it would spill over to all other nations, decreasing their population, especially among the poorer ones. Less people, less people to immigrate.
Do think the U.S could do this to their benefit? Maybe they could work in concert with China, as the Chinese are suffering from a female shortage, and there is a preference for boys. So it would help stabilize the population ratio of men to women (decreasing frustration and anger among young men, which could lead to social unrest, and possible revolution), and decrease the reproduction of the populace at large.