You may not attack members of your own team unless you play Chaos or unless it is the last week of the campaign (every man for himself…). 


Feel free to make alliances with nearby forces, for instance if you need to unite against a common foe. 


Use your common sense here, although some guidelines are laid out below.


Dark Eldar may do exactly as they damn well please, but backstabbing is to be encouraged. 


Kroot Mercenaries may ally with any force. 


Necons and Tyranids may only ally with other Tyranids and Necrons.

Chaos allies with CHAOS ORKS 
Orks allies with  ORKS but only after a battle is issued and BOSS is named
Imperials allies with Imperial, Eldar and Tau
Eldar allies with Imperials and Tau
Tau allies with Imperials and Eldar


Any other combination will be analysed by Lord Marshal.