Afghanistan peak US troops: 102,000 (2011)
Gulf war peak US troops: 700,000 (1990-1991)
Iraq peak US troops: range from 168,000 - 192,000 (2007)
Vietnam peak US troops: 543,000 (1969)
Korea peak US troops: 320,000, unclear what year
edit: obviously, Russia's involvement in Ukraine over the last few days would be by far the biggest operation in Europe since WW2. Just not globally (by a long shot).
I was too young to really get the scale of the first gulf war. Just transporting 700,000 troops seems astounding, let alone keeping them equipped in enemy territory.
https://en.wikipedia.org/wiki/Soviet%E2%80%93Afghan_War https://www.militarytimes.com/news/your-military/2016/07/06/...
presumably they've committed that or more than that to this operation since the navies are involved and it's easier logistically to get to
https://www.cnn.com/interactive/2014/03/world/infographic-uk...
But I agree with your skepticism, the world has unfortunately seen plenty of larger wars, mainly we've just had a short peaceful spell lately, and it's an unpleasant shock to stumble into another.