Ceasefire

After the (color) player activates a system that contains 1 or more of your units: The (color) player cannot move units to the active system. Then, return this card to the (color) player.

Promissory note

After the (color) player activates a system that contains 1 or more of your units:

The (color) player cannot move units to the active system.

Then, return this card to the (color) player.

Source: Base game

ClarificationsSelect rulings confidence level

tirules.com

The active player may still use the SPACE CANNON ability of any units during the Space Cannon Offense step of the tactical action.
If a player has units in the active system from before the tactical action in which their Ceasefire is played, those units will remain in the active system.
If the active player already has units with capacity in the active system, they may transport ground forces on planets in that system into the space area of that system. Those units may then be committed to invade a planet during this tactical action.
If a system containing only a player’s ground forces or structures is activated, that player may still play the active player’s Ceasefire.
Ceasefire does not prevent a player from producing or placing units in the active system.

Sources

LRR: Living Rules Referencetirules2: tirules2.comCommunity: TI4 Online Discord, TI4 Homebrew Hub Discord