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This strategy might have only worked with the destruction of cities by nuclear weapons in Japan.

The resolve of the German population was not broken by the bombing of civilians. If they wouldn't have hit the military production capabilities and invaded with ground forces the war would have dragged on much longer (and Germany lacked vital resources in their territory, unlike Russia).

So even if your suggestion to bomb the civilians wouldn't be quite reprehensible by itself, it's extremely unlikely that this would end the war on it's own.

Just look at the numbers of soldiers Russia has lost, this didn't seem to faze the support of the general population so far either (families and friends if those who died it who were severely injured).

So terrorism. You want to literally terrorize civilians for military gain. What is wrong with you?

Also, to add to the other poster's point, in a medieval siege, the defenders have every reason to believe the attackers will happily let every man, woman, and child behind the walls die gruesome deaths to starvatiom or disease. That's why, when it came down to the wire, cities would submit.

In modern times, cultivating a believable military posture of, "Surrender, or we will personally execute every last motherfucking one of you" is politically dicey. Look at the news stories coming out of Gaza about supplies running low thanks to Israeli interference. Right, wrong , or indifferent, the international community (as well as your domestic community, if those that disagree with these sorts of tactics are allowed to make their voices heard) tends to look down their noses at targeting noncombatants populations. So, due to these complications (which were largely absent or less impactful from warfare in the time of Genghis Khan) wholesale slaughter of civilian life isn't really openly used. In fact, guidelines like "proportionality" are invented which dictate the level of response you can give certain provocations and what not.

So, if you're a modern day commander being tasked with taking an urban center, the closest way to approximate a medieval siege would be to absolutely carpet bomb everything. Make it known that you will happily let every single person in Moscow die, if not send them to the afterlife yourself. While you're bombing the suburbs, you'll also need to encirce the whole city to prevent supplies from being delivered, since you can't guarantee every bomb will hit it's target and need starvation to provide additional assurance to the population that, if they maintain their current course, they are doomed.

Unfortunately, the world isn't going to allow that, and you know it, so you commit to the level of bombing deemed acceptable by the world at large. At best, you wind up in a situation like London during the Blitz. Your bombing runs are effective, in that they disrupt the daily life of citizenry, and cause some infrastructure damage and loss of life. However, you're never going to be allowed to scale up to the point where your victims feel they have no way out but to submit. There's enough plausible deniability that, even when a bomb hits close to home (literally or figuratively), the victim is more pissed at the bomber than their government.

Ye olde sieges cut off supply lines and forced the defenders to subsist on rations. Once those started running low, they started starving. Eventually the options were starve to death or surrender. These sieges frequently lasted months and sometimes years. Given travel times, it could also be weeks before anyone realized something was wrong and mobilized a force to break the siege.

Ukraine can only do infrequent drone raids. In order to properly siege Moscow, they would need to lock down all ways in and out of the city, and keep it that way for months, possibly longer given modern food preservation techniques and the viability of backyard farming. Additionally, sieging a city no longer prevents the people from communicating with the outside world, meaning other Russian forces would respond in days.