Does Grand Strategem apply retroactively to units that enter play after it is cast, or only to units in play when it resolves?

Ruling: Grand Strategem only affects units that are in play when the spell resolves. Units that enter play afterwards do not gain the +5 might bonus.

Sequence:

• Grand Strategem is cast and resolves
• All friendly units currently in play gain +5 might until end of turn
• The spell is discarded after resolving
• Any units that enter play after this point do not receive the bonus

Nuances:

• Spells cannot create passive effects that persist after they resolve and are discarded
• "Friendly units" refers specifically to units in play at the time of resolution
• The "this turn" wording indicates when the buff expires, not that it applies to all units played during the turn