Final answer:
The sentence emphasizing disarmament and resolving differences with intellect and purpose, rather than arms, introduces the need to replace acts of war with peaceful means of conflict resolution.
Step-by-step explanation:
The sentence from the passage that introduces the point that acts of war must be replaced with productive means of peacekeeping is: "Disarmament, with mutual honor and confidence, is a continuing imperative. Together we must learn how to compose differences, not with arms, but with intellect and decent purpose." This sentence emphasizes the importance of disarmament and the need for nations to resolve conflicts through intellectual and purposeful means rather than military strength. It suggests that peace is better maintained through mutual honor and confidence, rather than through fear and aggression.