Absentee voting (vote-by-mail) is of course universal, as no politician would want to be the first one to say "we're not going to let active military servicemen

Absentee voting (vote-by-mail) is of course universal, as no politician would want to be the first one to say "we're not going to let active military servicemen vote". That would be political suicide. Some States years ago opened-up absentee voting to literally everybody who simply asks for an absentee ballot. In some counties in those States, the percentage of people voting absentee was so high they just shut down the polling stations and have everybody vote absentee by default. Now that said, we are the US, and that means we have this thing called "the South", and people in "the South" want to have strict controls over the polling process and require ID because that makes it easier for them to exclude people who have the "wrong" skin color from participating. The US is a very large and very diverse country, and compared to European countries has very little central control when it comes to things like, for example, how elections are run. That's all State and county level.