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If both Formation are in the same hex when movement is concluded, then of course an engagement occurs with the following exception found in the Aerospace combat section on page 177: If an engagement occurs, both Squadrons will maneuver for
advantage. If the defending Squadron in an air-to-air attack has
not yet declared its own attack, it may decide whether it will return
the attack, attempt to move through the zone, or save its action for
an attack against a different target (such as an air-to-ground attack
if the engagement occurs in the Central Zone). If the defender
chooses not to return the attack when an engagement is initiated,
it cannot choose to engage its attacker later in the same turn.
The attack or Maneuvering for Advantage Roll that decides range and flank attacks against the side that fails it can only take place AFTER an Engagement Roll that determined whether an engagement takes place or not. If a Unit skips out to continue movement but DOESN'T and remains in the hex for whatever reason, then I would say that the initial failure for EngCR is overlooked and the aggressor gets the attack. If both sides did not commit to hostilities and they remain stuck in the same hex... they can awkwardly stare at each other for an entire turn for all I care--but the rules are clear to me. Only (see below) Aero must roll when they come into contact with enemy forces in the same zones of the ACS map. Yet you have the exception in the post above that contradicts the strict phrasing, which should supersede what was earlier written. For Formations on the ground AND in the air by pg177, the phrasing makes it seem completely optional to even pass on an EngCR roll.
pg.176 for Aero EngCR:If aerospace Squadrons from opposing Forces occupy the same
zone at any time during movement, Engagement Control must be
determined in order to see if combat occurs.