When a team-member withdraws his troops or transfers his troops to his partner, it is obvious that those remaining "soldiers" would offer no resistance. It would be a much more strategic game, also more balanced, if team-members were simply allowed to conquer each other's territories without rolling the dice (or fixing them so that the defending player always loses).
Example.
Steve transfers his 2 units from Iceland to his partner in Great Britain, so that his partner can conquer Europe. His partner now has 5 troops on Great Britain and decides to attack remaining unit on Iceland. Logically, troops in Iceland should OFFER NO RESISTANCE, because they would WANT their partner to win. However, game is made in such a way that they do offer resistance and this is not logical.
Solution:
Fix the dice so that defending team-member always loses (allows his team-mate to win), or eliminate dice-rolling in such a case altogether.