The USSR and even China took over quite a number of nations quite successfully, the USSR has the entire Eastern Block under it’s iron curtain, subdued by military force. Poland, Bulgaria, East Germany, Hungary, Romania, Latvia, Lithuania, & Estonia. Czechoslovakia too, although the Soviets had more problems there, earlier. All were conquered by the uSSR and remained part of their empire for over 40 years.
Germany, during WWII, had taken over most of Europe. Same for Napoleon. Rome had almost the entire civilized world under the SPQR banner, for seven centuries, and in some areas even longer.
The only reasons why the USSR couldn’t hold Afghanistan is :
- it had a fanatic terrorist and guerrilla population.
- It had a lot of outside help, at least for munitions
- The USSR was overstretched.
Now sure, it’s very hard to conquer a nation with a well armed fanatic terrorist and guerrilla population, especially if there’s help from the outside. But Kuwait rolled right over.
Your idea that a nation can’t be conquered is simply wrong.