Arnhem is a legitimate victory city even though it doesn't have an Axis Flag, correct?
May you place an Axis marker in Arnhem?
Yes, Arnhem is a victory city
. (see rulebook, page 2, description #3).
The lack of a printed Axis flag indicates only that it can't be used as a location from which to spread an Axis Reaction. See the placement rules for Axis Reaction on page 12. Unless
However, as long as you can trace a path of Axis markers back to Dusselforf (such as having Axis markers on Kleve and Rheinhausen) there's nothing preventing you from placing an Axis marker on Arnhem
. Then in that case, you could spread an Axis Reaction from Arnhem, but it would be moot as there'd already be a marker in Kleve which would serve the same purpose.
I assume you're asking if it's possible to place an Axis marker there, only to prevent from having to place the marker elsewhere (otherwise from a gameplay perspective, an Axis marker in Arnhem has no effect if there's already one in Kleve). From my interpretation
of the rules, I'd say it's perfectly legal to place a marker in Arnhem as long as you follow the rules for placing markers!