The war began when the Arab coalition launched a joint surprise attack on Israeli positions in the Israeli-occupied territories on Yom Kippur, the holiest day in Judaism, which occurred that year during the Muslim holy month of Ramadan. Egyptian and Syrian forces crossed ceasefire lines to enter the Sinai Peninsula and Golan Heights respectively, which had been captured by Israel in the 1967 Six-Day War. Both the United States and the Soviet Union initiated massive resupply efforts to their respective allies during the war, and this led to a near-confrontation between the two nuclear superpowers.