[Fate]

Stealth has no effect on troop movement. The only thing that matters in retreats is province OWNERSHIP. The presence or non-presence of troops, stealthed or not, has no effect.

If you control a province your opponent may not retreat there, if your opponent owns it they can retreat there.

The most painful part is that stealth troops may not retreat into stealth. So if you attack in the middle of nowhere with no province to retreat to it is all too possible to lose your army because of no retreats.

You might, as Falkor points out, need to fight the same army twice if you use magic attack. There is, however, no combination of normal movement (with or without stealth) that would cause you to fight the same enemy army twice.