2004-10-04 : If a spell is modal, use the target(s) of the current mode only for this check. Targets for the other modes are never selected.

2004-10-04 : If the spell’s target leaves the battlefield before this spell resolves, Intervene won’t resolve since it no longer targets a spell that targets a creature.

2004-10-04 : Intervene can only target a spell that is currently targeting a creature.

2004-10-04 : It means “targeting at least one creature” not “exactly one creature”.