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Only peace can save the world.
Literal
Peace only world [object-を] save [thing-こと] [topic-は] cannot.
平和しか...できない uses the しか~ない pattern: 'nothing but peace can do it.' This structure means 'only X' by negating everything except X. 世界を救う is 'to save the world.' An emphatic, almost slogan-like declaration.