I must think over my position and how I may improve it.
1- Suppose I want to attack unit X with both my units A and B. Unit A rolls 2 flags and unit X retreats out of reach of my unit B. Now what? Do I:
a) Lose my chance to combat at all with unit B
b) Choose another valid target for my unit B
c) I should only decide my target for each unit when it's its turn to battle (instead of deciding the target of each unit before the start of the combat phase)
C. Although you can sometimes lose your chance to combat still.
2- I know that I must remove the sandbag when the initial unit leaves. What if:
a) In the same turn, I move unit A out of sandbag hex but immediately replace it with unit B? Sandbag gone or not?
Sandbags only apply for the unit that started there, iirc.
b) Enemy successfully attacks and kills my sandbag unit with a close combat attack. He decides to advance on my hex (taking its ground option). Sandbag gone or not?
See answer above.