[1] After graduation, Trainin worked in the MGU Department of Criminal Law, on track for a professorship, but he would resign his position in 1912 in connection with the "Kasso Case" [ru], in which a great many academics resigned out of solidarity with the targets of Imperial Education Minister Lev Aristidovich Kasso.

Trainin came to prominence in the inter-war years as critical of the League of Nations for not doing enough to prosecute those who waged war against peace.

He proposed that a new legal concept, "the crime of aggression", be used to hold Nazi Germany's military and political leadership accountable for the numerous countries they invaded and occupied.

Along with the other jurists involved in crafting the Nuremberg Charter, Trainin was influential in establishing the new legal field of international law.

[1] Despite this foundational role, his contributions are often ignored or forgotten by Western scholars, largely as a result of Cold War perceptions of the Soviet Union.