Though the Convention for the Prevention and Punishment of the Crime of Genocide was officially adopted by the United Nations in 1948, it was still necessary for 20 nation-states to ratify the convention before its codification as international law.

Just as he had lobbied tirelessly for the Convention’s adoption, Lemkin devoted himself to securing its ratification.  His strategy was to try to obtain ratification status from countries on each continent, hoping to cause a “domino effect.” For instance, he lobbied Mexico in the hope that the rest of Latin America would follow; and he looked to the Philippines as the key to success in Southeast Asia. He sent countless letters, and telegrams, made phone calls, and met with anyone who he thought could help further these goals.

By 1951, 25 nation-states had ratified the treaty, and the Convention was officially introduced into international law.  Since then, the Convention has been ratified by a total of 140 countries.  

Lemkin was disappointed that his adopted country, the U.S., was not among the first to ratify the Convention. In the intensifying atmosphere of the Cold War, anti-UN sentiment and concerns about loss of sovereignty played a strong role in the opposition. Some worried, for instance, that the law would permit the extradition of U.S. citizens to foreign countries or that discrimination and violence against African-Americans could be considered genocide under its provisions.

In fact, ratification by the U.S. did not occur during Lemkin's lifetime. It was only in 1988, under President Ronald Reagan, that the U.S. ratified the convention, largely through the efforts of Democratic Senator William Proxmire, who, over a 20-year period, gave over 3,000 speeches in the Senate lobbying for the passage of the Convention.