The term "Divide and conquer!" refers to the strategy of splitting your enemies forces, and taking them out one at a time with your full force. Why do you think we are Chaos Undivided?
Chaos is chaos and it knows nothing of honor only victory. Make pacts with the Orks and maybe other factions, concentrate forces on acquiring short-term objectives responding to a long-term or medium-turn strategical plan. NightLords can be a perfect skirmishing or indirect force, disrupting enemy lines, ambushes, getting control of far off strategic points etc, Black Legion and Word Bearers should be the most flexible forces of Chaos meaning they would take care of the general fighting and act as support when required. Iron Warriors should take care of siege related warfare. But most importantly EACH faction should base their whole plan and strategy on the greater and long-term goals the chaos faction plans to do. This means absolute support between all subfactions. As an example: lets say a base is heavily defended by the enemy and enemy reinforcements are on the way, Night Lords with some components of the BL or WB could disrupt reinforcements and/or waylay enemy outlying positions to further saw confusion, meanwhile IW take care of the siege while BL or WB or both come in as supportive roles filling in necessary requirements as the siege goes on. When the base is taken it would further better the overall strategic position of the faction and then consolidate towards the next goal until the freaking planet gets raped by chaos. Cooperation is of extreme importance in this game, and as in any army obedience is expected of all soldiers and warriors, THE CHAOS GODS DEMAND IT. This applies to all factions. I will probably play as Chaos as i'm part of a chapter (clan) from the Space Marine game and we plan to "transplant" the chapter over to EC (with the possibility of joining Chaos being very great) to beat those imperial scrubs.