The "Marx-Engels Conflicts" theory created by international academia, and the consequent inference that Engels was not a Marxist, has certain repercussions in the field. Accordingly, defending Engels is not alarmist. This paper reveals the intellectual cooperation and theoretical complementarity between Engels and Marx by following the thematic logical evolution of three main themes: philosophical transformation, social criticism and economic analysis. In contrast to the usual "rupture" development mode between idealism and materialism, the contribution of Marx and Engels lies in the fact that the above three main themes are interlinked, interpenetrating and inter-accompanying. Any significant principles of Marxism must be accompanied by references to the ideas of Marx and Engels at different times and in different texts, otherwise it would be out of context to express Marxist principles in isolated propositions. Therefore, for the change from "polar opposites" methodology to "theoretical complementarity" one, defending Engels is essentially defending Marxism.