From some cursory googling:
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).
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).