'Sentry Mode' for Defending Armies

TL;DR: A toggleable army stance (similar to Air Patrol) that immobilizes armies but lets them automatically attack armies that move adjacent to them (similar to Ambush)

One quirk of simultaneous turns with AI is that when moving an army adjacent to theirs, they will immediately trigger an attack against it (if they think they can win, of course). Some players have complained that this feels unfair, since the AI will always react faster than a human, but perhaps this could be made into an actual feature.

An Army that has not moved or acted yet on a turn would be able to activate a special stance, which I'll be calling Sentry mode. While in Sentry mode, the army will automatically attack any army that moves adjacent to it, similar to the Ambush feature of Stealth units. This will allow you to gain the benefits of first strike even while defending, but only while defending secured territory. Requiring full movement means you can only activate Sentry mode by moving your army into position the turn before, making it only feasible to use when defending your territory, or securing gains after a major skirmish.

Flavor-wise, this takes the form of an army that's been instructed to dig in and wait for any enemy movement. It represents the time taken to fortify a position, letting the troops scramble while the enemy cavalry races over the hills, so to speak. Preferably, armies in Sentry mode would have either a unique icon or some other effect making it clear that they're in this state, so it can be played around. Stealth units would also find another role here, as if they make it to an army in Sentry mode undetected, they would be able to trigger an attack with the first strike advantage as usual.

As a more nuanced tactic, it might make sense to lock it behind a technology similar to Reinforcements. Perhaps Standing Army (fits the flavor) or War Summons (introduces the Sentry tactic alongside its first counter, the Prowler). As far as balancing, armies in Sentry mode could have increased upkeep, require spending all of their movement again to exit Sentry mode, be unable to Reinforce nearby battles, or any combination of these.