The Mao quotation reads:
"The people of the world must unite to oppose any war of aggression caused by imperialism or social imperialism, and in particular to oppose a war of aggression that makes use of nuclear weapons! If such a war starts, the people of the world must counter this aggressive war with a revolutionary war, and must start to prepare for it today!"
Nr. 2 of the set 'Posters showing measures against atomic, chemical and bacterial warfare' (1971).