Russia is the bigger problem currently, most likely. China isn't trying to conquer its neighbors, just one of them, and also the South China Sea. The Russian people, however, really do want to re-establish a Russian empire of some sort. They don't really have the economy or means to do so, but they do have a bunch of nukes (which may or may not be fully operational) to cause a lot of damage with, and they have a geriatric dictator with possible mental problems who they wholeheartedly support.

I’m not sure China was ever particularly interested in conquering foreign countries now or historically (not saying Taiwan is not a real country but it’s not really foreign and certainly an exception in most ways).

On the other hand violent imperialism has been a core aspect of Russian culture for centuries.

No fly zones are enforced on a country by shooting down military aircraft. It sounds alluring, as it’s not called a war or invasion. But in order to do that you first have to make it safe for your own military jets by using cruise missiles on air defense systems, airbases, command posts, telecommunication hubs, etc. It’s inseparable from starting a war. A sovereign state exercises control over their territory. To deny them the ability to use forces in their own territory you have to engage in acts of war.

As seen in Libya our governments proceeded to expand their mission from the UN mandate to regime change. Obama danced around the War Powers Act and the bad optics of starting a war by calling it a “kinetic military action” lol.

That was the miscalculation. Conquering territory fast isn't good enough to achieve victory in war. In fact the opposite is probably true: being able to steamroll your immediate neighbors can cause you to overextend and ultimately lead to defeat.

> It is only after the horrors of WW2 that the western world became fearful of casualties

That’s arguably not true at all. At the beginning of the war the allies were very reluctant to engage in major offensive operations (in hindsight it’s not inconceivable that the French could have won in 1939 while Hitler’s army was busy in Poland had they been more risk tolerant). The French especially were very fearful of casualties (in WW1 it lost a larger proportion of its male population than even Germany and France had relatively very low birth rates historically making them much harder to replace)

That makes sense considering that most of the people in charge were field officers in WW1 and knew the cost their decisions might have. Of course that changed over the next few years since it turned out that there were no other options.