Yeah, at the peak of Vietnam we had more troops on the ground than the Army has on active duty today (about 450k). The Army's size declined by half after Vietnam, and then declined by half again in the 1990s. This is a nice figure illustrating active duty end-strength for all services since 1950: https://www.laits.utexas.edu/gov310/media/fad_milper_400.gif

That figure doesn't go back as far as WWII, of course. Even the Korea and Vietnam-era Army would be dwarfed by the WWII Army: 8 million troops in 1945.

Note that we do also have the Army Reserve and Army National Guard, which effectively doubles the total Army end-strength to about 1 million today.

As a career Army officer, I could go on about this, and what it means, and what worries me, all day.