Polish writer, educator, diplomat, and jurist who profoundly influenced the postwar development of international law.

Lachs was educated at Jagiellonian University of Kraków, where he earned his law degrees, and did graduate work at the Consular Academy of Vienna and the London School of Economics before the outbreak of World War II.

His first public notice in the West came in 1945 with the publication of his first book, War Crimes: An Attempt to Define the Issues. Lachs was made a delegate to both the Paris Peace Conference and the first United Nations General Assembly (1946). The following year he was appointed director of the Legal and Treaties Department of the Foreign Ministry, a post he held until 1960. In that year he became legal adviser to Foreign Minister Adam Rapacki and played a central role in the development of the “Rapacki Plan” for making central Europe a nuclear-free zone. Lachs was a delegate to most General Assembly sessions through 1966. In that year he was elected a judge of the World Court, formally the International Court of Justice, at The Hague. He was president of the court in 1973–76 and was chairman of its committee on revising court procedures.

Throughout his political and legal careers Lachs continued to teach and lecture around the world; from 1952 he taught at the University of Warsaw. He published several books, including The Teacher in International Law: Teachings and Teaching (1982), and many articles.