For those who don’t know, Truman was a Redleg (artilleryman) in the Missouri National Guard, then the Army Reserve. Before the 1950s, it was known as the Organised Reserve Corps, the difference being that MOARNG is, of course, a State militia. When WW2 broke out, then-Senator Truman wandered over the Marshall and volunteered for duty. At age 56, Marshall declined the service of “the old stiff”

The reserves constitute several distinct categories. See https://fas.org/sgp/crs/natsec/RL30802.pdf In Truman’s case, he would have been part of the IRR equivalent. Note the use of terms like “may” and “can”, there is no requirement that a member of the IRR actually be called to duty. So, as long as the President said that the Senators and Representitives didn’t need to be called up, they could happily stay in reserve. There are currently 16 members of Congress who are reservists, DoD Directive 1344.10 of 2008 lays out the current guidance and explicitly permits reservists to run for political office. I’m not sure what the specific regulation force in 1940 was.

Rank inversion is not unheard of even today. On my last deployment, the Lt Governor of Guam joined us. He had the rank of Colonel, he was our surgeon. Basically, Monday to Friday, he outranked everyone in the Guam National Guard. On weekends or for training, he was subject to the orders of the senior staff who would ordinarily be subject to his civilian control in the absence of the Governor. It seems to have worked out.