1917-Third Battle of Ypres: Canadian troops recapture the high ground in the village of Passchendaele from the Germans in West Flanders, Belgium, during World War I; over 200,000 casualties occur on each side, and 24,000 German prisoners are taken 1948-Earthquake in Ashgabat kills 100,000 in Turkmen Soviet Socialist Republic 1948-Paleoanthropologist Mary Leakey finds the first partial fossil skull of Proconsul africanus, an ancestor of apes and humans, on Rusinga Island, Kenya 1949-U.S. President Harry Truman signs the Mutual Defense Assistance Act to strengthen NATO allies especially Greece and Turkey 1951-Joseph Stalin announces that the Soviet Union has an atomic bomb 1956-Scientist Albert Sabin announces that his oral polio vaccine is ready for testing; it soon replaces Jonas Salk's vaccine in many parts of the world 1976-Mao Zedong's widow Jiang Qing, Yao Wenyuan, Zhang Chunqiao and Wang Hongwen known as the "Gang of Four" are arrested and charged with plotting a coup, an event often considered the end of the Cultural Revolution 2021-WHO Director-General Tedros Adhanom Ghebreyesus reco