I don't see where you got supply into this. It is strategy to plan that you will move your units to Tile X via Route Y. To simply do that. Plan the route of your invasion. It is tactics to alter the minutiae along the way of that route, thereby effecting outcomes that would not otherwise be effected.
I'd still say that is tactics, given that the strategy involved would be having a defensive frontier, rather than using a particular tile and game mechanic as that frontier that is strategically decided upon.
It would be tactical because you could decide for one particular battle to group your units as an army of not, depending on your situation.
Have to say that not much of this has to do with the OP. You'd have your army grouped together as much as possible because units on the same tile defend as a single army and thus you'd need all your units to capture a city, for instance. You'd only keep your army spread out as a strategy to avoid being surrounded or to reduce dependence on supply units.