If Stand United is hidden on battlefield A, can I react to an attack on battlefield B by playing it to buff a unit on A, and will the +1 to all buffs effect also apply to units on battlefield B?

Ruling: Yes, you can react with Stand United from battlefield A when battlefield B is attacked. The direct buff targets a unit on battlefield A (where it was played from), but the +1 to all buffs effect applies board-wide to all battlefields.

Sequence:

• Stand United is played from the facedown zone at battlefield A
• The "buff a friendly unit" effect becomes "buff a friendly unit here" (at battlefield A only)
• The "+1 to all buffs to friendly units this turn" effect applies globally across all battlefields

Nuances:

• When played from the facedown zone, the direct buff is restricted to the battlefield where Stand United was hidden
• The secondary effect that increases all buffs is not restricted by location and affects all friendly units regardless of which battlefield they're on