What must an actor believe to justify using force to protect a third person?

To justify using force to protect a third person, the actor must have a reasonable belief that intervention is immediately necessary and that they themselves would be justified in using force to protect their own person in a similar situation. This involves assessing the circumstances and determining whether the third person is in imminent danger of harm. The key aspect here is the immediacy of the threat and the belief that force is a necessary response to prevent injury to the third person.

This understanding aligns with principles of self-defense and the defense of others, where an individual may act to protect another from substantial harm as long as their beliefs about the necessity and the justification for using force are reasonable under the circumstances. Thus, an actor who reasonably believes that they might justifiably use force on their own behalf would extend that justification to protect someone else who is in similar perilous situations.