When approaching enemy units stop your group just before you reach range and allow the ai to kick in and handle the combat. Small groups can be grabbed by a competent player and focused on units needing a priority the AI disagrees with ( most units will attack an inactive turret at cost of being hit by an active tactical unit like a tank - or even walls!!).
Anytime a unit is moving it stops firing so continual fiddling will result in your units not firing much compared to the enemy - thus dying even if outnumbering the enemy.
The defensive settings are an aspect of combat which the typical RTS player may not be used to. This button is the rightmost button with a unit selected. These settings allow for advanced tactics when you are offline or not paying attention. They also allow the units to discriminate their targets, from return fire only to attack everyone that gets too close. Keep in mind that political stance is set automatically if fired upon, therefore, you don't necessarily want to make everyone that comes by your base an enemy by setting your units to "anyone who approaches" though, at times, it can be useful. Considering these settings can also be important when attacking as moving units that are not your own could set off those settings.
