There are two steps for NCUs.
The situation in question looks like this
- Claiming a zone
- trigger zone effect
- Player A claims Horse
- Both players declare tactics card based on trigger “NCU claims horse”
- Player A declares first, then player B
- Player A uses card first, replacing zone effect with endless horde. Card is resolved. Also apply any influence abilities at this time.
- Player B uses card next, replacing zone again with field control. Card is resolved.
- Player A resolves the zone effect (field control in this case)
Ah, so you fall into the same side of the Timing Sequence argument that my playgroup does. However, tournament organizers and the rules group on Discord (made up of many of those Tournament Organizers) have said that Zone Resolution is a "When the zone is claimed" trigger, just like Influence, NCU abilities, and Tactics Cards. So Zone Resolution is a simultaneous effect, meaning the active player resolves all of their cards, abilities, *and the zone effect* before the Targ player would resolve Field Control. This wouldn't be a big deal if Field Control's ability said "Before resolving the zone's effect, you may force your opponent to replace...", but it doesn't. Because it doesn't specify a different timing, it operates on the simultaneous effects timing.

