Under US copyright law, anything created by the US government is not protected by copyright. It is automatically public domain. (There are some complexities to this that we could get into but they're not relevant here.) But it doesn't necessarily follow that every other country in the world has the same laws.
In the case of the Soviet Union, the Soviet Union no longer exists, so it's not clear who would sue you for infringement! In any case, current Russian law specifically excludes the text of laws and government symbols from copyright protection. So you could freely copy contemporary Russian military insignia. Even if Russia considered itself to have inherited documents and symbols from the Soviet Union, they would presumably fall under the same exemption.
As Cyn says, if you are creating a fictional country, you probably don't want to copy too much from a real country, as that could quickly become distracting and out of place. If I saw a movie based on Lord of the Rings and the hobbits had American flags flying over their village, my first thought would be that there was supposed to be some connection between the fictional hobbit nation and the United States, that they were eventually going to reveal that this place was founded by inter-dimensional travelers from the US or something. And if no such connection was ever introduced, I'd think the producers were either really messed up or were trying to make some kind of point.