Russia: Invaded with less than 200k soldiers. They later on mobilized approximately 300k soldiers. This mobilization was extremely unpopular and resulted in mass resistance within Russia, Russians emigrating and so on. Since then they swapped entirely to a 'voluntary' system of deployment, with some reported incidents of coercion. Benefits for soldiers are extremely high which is likely driving a significant rate of enlistment. Soldiers are demobilized after their contract ends, which is a very important issue for a war entering into its 4th year.
Ukraine: Had approximately 300k soldiers before the invasion. After the invasion they closed their borders, prevented men of 'fighting age' from leaving, and declared a general mobilization so that anybody between the ages of 27-60 could be forcibly conscripted and deployed to fight. They have carried out this mobilization very aggressively as well as constantly lowered the minimum age, and standard, of mobilization in a country that already had a severe demographic crisis before the war. Demobilization is somewhere between inconsistent and nonexistent.
Current: Both sides seem to agree that Russia has a significant manpower advantage with an army of approximately 700k soldiers remaining in Ukraine.
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To even begin to create comparative casualty measures you need to create estimates for how many soldiers Russia was able to 'voluntarily' enlist and estimates for their contract length to account for demobilization. And then you need to contrast this against how many people Ukraine was able to conscript and mobilize. You can find numbers for these online, but nobody's even pretending to try to be remotely objective and there's highly organized propaganda abounds, so the numbers are completely meaningless.
And then on top of that you also need to somehow figure out how many are actual casualties and how many are desertions. For instance Ukraine has apparently filed more than 300,000 criminal cases over desertion. Of course they also have numerous major motivations to avoid KIA classifications. So basically yeah - you're not going to be able to realistically estimate casualties on either side, with anything even resembling a reasonable degree of confidence, until the war is over.
Ukraine is going to have some painful demographic issues to deal with when the dust settles (and I am cheering for them!).
The scariest thing is that even in the best-case scenario, this may no longer be possible. Even before the war, Ukraine's demographics were dire, then young people left, and no matter how the war ends, there's no objective reason for them to return.
On paper the US is broke. I'm not sure about the Europe situation but I doubt they're doing well either the growth statistics I saw last were a joke. They're all struggling to even arm Ukraine for this war.
If I were a Ukrainian strategist I too would not count on some sort of vast infusion of resources appearing after the war. These are not the conditions of the Marshall Plan where Europe had massive economic potential and the US was the world's leading industrial superpower with wealth to spare.
As you said Ukraine’s demographic situation was quite horrible before the war. Very few people in their 20s. Hence the conscription age being 27 earlier in the war. They lowered it to 25 later (which is kind of the inverse of what happened historically in other wars).
Russia had way more manpower, then the cannon fodder from North Korea and the foreign mercenaries. Russia can afford even 1:1.5 or 1:2 casualty rates (of course they have other concerns and seemed to be very politically unwilling to send actual conscripts there and the pool of willing volunteers is not infinite).
This is a perfect situation for waging war. Young people are prone to rebellion and overthrow the authorities that send them to war.
> Hence the conscription age being 27 earlier in the war. > seemed to be very politically unwilling to send actual conscripts there
It is exactly because of that reason. The younger the people, the more dangerous they are for the government.
Most people don't realize is that war is essentially a giant deadly game of logistics, and so the typical plan for Russia would be to simply destroy the logistics pipelines arming Ukraine. But thanks to the people 100% responsible for maintaining Ukraine's military managing to maintain a strategically accepted neutrality, it's impossible to fundamentally disrupt their logistics pipeline outside of small scale black ops stuff.
So that has turned this war into a war of attrition where Russia is advancing slowly, but mostly setting the goal as essentially having Ukraine simply run out of Ukrainians. And they seem to be succeeding. Once the real death tolls for this war are revealed, people are going to be shocked. You don't need to drag in people off the streets, close your borders, and continually lower the enlistment age (in a country with a severe demographic crisis) if you're not suffering catastrophic losses, especially since as the amount of territory you have to defend decreases, you need fewer soldiers to maintain the same defensive density.