The Arab League members Egypt, Transjordan, Syria, Lebanon and Iraq refused to accept the UN partition plan and proclaimed the right of self-determination for the Arabs across the whole of Palestine. The Arab states marched their forces into what had, until the previous day, been the British Mandate for Palestine, starting the first Arab–Israeli War. The Arab states had heavy military equipment at their disposal and were initially on the offensive (the Jewish forces were not a state before 15 May and could not buy heavy arms). On 29 May 1948, the British initiated United Nations Security Council Resolution 50 declaring an arms embargo on the region. Czechoslovakia violated the resolution, supplying the Jewish state with critical military hardware to match the (mainly British) heavy equipment and planes already owned by the invading Arab states. On 11 June, a month-long UN truce was put into effect.