Hi Horus40K. Welcome to the forum.
A dead zone is a territory or sz sandwiched between opposing forces. Neither sides wishes to place his “stack” of units into that dead zone given the risk of being blown away by his opponent.
Nevertheless both sides perceive an advantage to at least temporary ownership of the dead zone, perhaps because it acts as a barrier to the opponent’s movement, or to deny use of a factory, or to draw opponent’s forces from elsewhere, or sometimes to gain the economic value of the territory.
Given that the dead zone is captured merely to be lost again the minimal force is best. However, judging that minimal force can be nuanced.
For example, a 1 fighter + 1 infantry attack on 1 infantry, or 1 fighter + 1 destroyer attack on 1 destroyer, has a c. 90% chance of success according to the triple a battle calculator. Success means killing the enemy whie having at least one unit left yourself. So if that success includes losing your own infantry or destroyer, you do not gain ownership of the dead zone at the end of combat.
This is less of a problem at sea because an empty sz can be occupied during non-combat if you wish to place a barrier to opponent movement.
On land, however, you cannot occupy a hostile albeit empty territory during non-combat and so have allowed your opponent use of the factory, retention of the territory’s economic value, the opportunity to move tanks through to a territory beyond, or land air power there on his next turn. The importance of these things will determine what you consider to be the minimal force you allocate to the attack. (NB The ability to land air power may be enough to turn the territory into one that can be defended on the next turn, given the defence power of fighters, resulting in the whole front moving)
One thing you will have noted from the above is the importance of air, given that it does not land in the captured and threatened dead zone, but retreats to safety. Only air can do this.
I do hope this all made sense. I have no doubt that others will add to this explanation and correct any errors I may have made.