Kelsen attempted the most rigorous version of legal positivism ever constructed, and also drafted a constitution and invented an institution that most democracies now have.
The pure theory is pure in a specific sense: it excludes from legal science everything that is not law. Sociology explains why people obey; ethics judges whether they should; psychology describes what officials feel. None of these tells you whether a norm is valid, and validity is jurisprudence's only proper object. A norm is valid because it was created in the manner a higher norm authorizes — a bylaw under a statute, the statute under a constitution — so the legal order is a hierarchy, not a set of commands. At the top the regress must stop somewhere, and Kelsen stops it with the Grundnorm: a basic norm that is not itself posited but presupposed by anyone who treats the order as binding at all.
The consequences are austere. The state is not something separate that makes law; it simply is the legal order viewed as a unity. And validity is entirely formal: a monstrous statute properly enacted is valid law, which Kelsen accepted as the price of a science that does not smuggle the theorist's politics into its definitions.
He drafted the Austrian constitution of 1920 and, in it, created the first standalone constitutional court with the power to annul legislation — the "Kelsenian" model of centralized constitutional review now used across Europe, Latin America and Asia, and distinct from the American diffuse model. He served on that court until removed for a politically unwelcome ruling on the annulment of marriages.
Jewish and a social democrat, he fled Germany in 1933 and Europe in 1940, finishing his career at Berkeley.