Move order question

[Sombre]

What happens when units from province A, owned by nation Y, invade province B, owned by nation Z and in the same turn the troops from B invade A. Do they switch over, or does combat occur?

[Edi]

Combat, but which province it happens in is arbitrary.

[Nick_K]

I think the bigger army is supposed to have a better chance of 'pushing' the combat into their target province