Here's why:
* Subjunctive mood is used to express wishes, desires, hypothetical situations, or commands.
* The sentence expresses a desire for world peace, which is a hypothetical situation.
* The subjunctive mood is marked by the use of the base form of the verb ("be") even though the sentence is in the present tense.
Example:
"I wish I were a bird" (Subjunctive mood, expressing a desire)
Note: In modern English, the subjunctive mood is becoming less common. The use of "be" in this sentence might sound a bit formal or old-fashioned, but it is still grammatically correct.