US military Retired List - can they be reactivated?

[Grey Wolf]

I read biographies of US military personnel (army, navy, marines) and get to a point where they are "placed on the retired list".

One of these, however, Admiral Thomas C Hart is listed as having continued to work for the Navy Board for 3 years after he was so placed

Does this imply that if a president (or senior military commander) wants it, he can request (& require?) someone from the retired list to continue in service, or to hold a new position?

Were any commanders on the retired list un-retired at the outbreak of WW2 for example?

I'm also confused about MacArthur - looking at the ages that other generals were placed on the retired list one would have expected him to have been in 1946 or so, but he seems to have remained in active service into the 1950s and thus into his 70s?

Best Regards
Grey Wolf