Einstein was a socialist with a small "s," and he never espoused anything like a consistent "party line," yes, he opposed militarism (he was a pacifist) and imperialism and the deployment and use of nuclear weapons. He allegedly did express some regret in his small role in the development of nuclear weapons though some of this has been exaggerated in later mis-quotation (he never said, for example, that atomic weapons made him wish he'd have become a watchmaker — he actually said that if he were a student again today, he instead decline to go into physics and would go into something that might afford more independence, a somewhat different sentiment altogether). I would not characterize either his science or his wartime letter-writing as actually being all that essential to the development of the atomic bomb, but he did leverage his fame and connection to the matter to promote his political views, notably to argue for world government, international control of nuclear weapons, disarmament, etc. The most prominent example of this activism is his signing, shortly before his death, the Russell-Einstein manifesto.