On April 7, 1967, Syria executed terrorist attacks on Israel. The attacks were directed at an Israeli tractor working land in the demilitarized area on the Syrian–Israeli border. Syria and Israel exchanged fire all day. At the end of the battle, Israel had shot down seven Soviet-made Syrian aircraft. This was the first air battle between the two nations. The USSR supported Syrian attacks and blamed the violent acts on Israel. Syria did not hesitate to act because it believed that the other Arab states would support it and Israel was not capable of defeating it. In the UAR, the USSR motivated Nasser to have the UN forces leave the Sinai Peninsula and the Gaza Strip and to blockade the Straits of Tiran. Like Nasser, the USSR didn't believe that Israel would start a war on its own. Even if Israel did attack, it was unlikely that Israel would be capable of defeating the Arab states. Syria believed that, with the help of the UAR, it could beat Israel. On May 11, the USSR warned the UAR that Israel troops were gathering on the border with Syria and that an invasion was planned for May 18 to May 22. At this time, the USSR also began to publish accusations against Israel in order to cement the defensive unity of the UAR and Syria.