Also, to actually address OP:
>Some places I came across said we had over 3.5MM military personnel in 1968. Post-9/11 we didn't see a major increase in levels, levels have kept a little above or below 400k since then.My understanding is USA fights proxy wars where possible.
How many US soldiers have died in Ukraine? Wikipedia currently lists
'under 50.[1]
You actually don't need to send your own citizens and military to the slaughter if you are an imperial power. And China is the US's biggest threat, so they still want to show an ability to punish dissenting nations using Ukraine (after their failure in Afghanistan, Russia is an important showcase of NATO's enforcement power, which would make it a devastating battle to lose) to chill the Asia-Pacific. But the US can keep their military and citizenry available for fighting China.
But, in a situation where they're fighting China, I don't see their current military being nearly sufficient and militarizing a proxy nation probably wouldn't be enough, so a draft is a real potential outcome if the war doesn't stay cold. In an ideal world, we could just get all the nationalist sinophobic scum to volunteer and get them purged first. But I don't think the facade of freedom in the US could handle regular civilians being blackvanned, that's too blatant and over the line for The Land of the Free until it's in existential crisis.
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[1]
https://en.wikipedia.org/wiki/List_of_Americans_killed_during_the_Russian_invasion_of_Ukraine