On September 13, 1973, an air battle took place between Syrian and Israeli fighters. It was an alarming skirmish in which twelve Syrian aircraft were shot down while the Israelis lost only one fighter. Tension ran high between both countries. The Egyptians in particular were very concerned; on April 7, 1967, an air battle between Syria and Israel had escalated the military situation and was one of the causes of the Six-Day War. Aware that war was to be launched only days later, the Syrians chose not to retaliate. The air battle aided the Syrians in concentrating their forces for the war, as the Israelis interpreted it to be a defensive reaction on the part of the Syrians. The Israelis closely monitored this buildup near the front, but their intelligence remained adamant that Syria would not go to war without Egypt, which the Israelis believed was currently occupied with internal issues.