Basically every unit (army or navy) has an attack/defense value of one, which means any two units moving into the same space cancel each other out. The only way you win is by supporting your own units’, or other players’ units’ moves. Therefore 1 unit moving into a territory of another, with the support of a 3rd adjacent unit, will win if the second unit is unsupported.
The kicker is that all moves are simultaneous, and you can never know what another player is actually planning to do until everyone moves. So your “ally” could be faithfully supporting your moves–or stabbing you in the back by helping himself or someone else, or moving in where you vacate, or just sitting back and letting you fail. That’s where the “Diplomacy” comes in…
TG is right about the numbers–we usually play “Diplomacy” when there are too many prople for an A & A game, sice “D” works best with 6-7 people. The secret alliances/ententes/grudges are best with these numbers…
Ozone27