Task: Question Generation
Input (English): Passage: The month-long truce went into effect on 11 June, 1948. On the same day, the first group of 36 observers arrived via Cairo, Egypt and continued to arrive for the next three days. The first truce did not last long due to widespread violence which again erupted. As a result, the observers were withdrawn on 9 July 1948. The second truce, indefinite in length, was called by the United Nations Security Council on 15 July 1948. This declaration was to be put into effect on 18 July 1948. It was from United Nations Security Council Resolution 54 that the Mediator was instructed to supervise the observance of the truce and to establish procedures for examining alleged breaches of the truce since 11 June 1948, and authorized the Mediator to deal with breaches so far as it was within the capacity of the Mediator to do so by appropriate local action, also the Security Council Resolution 54 requested the Mediator to keep the Security Council informed concerning the operation of the truce and where necessary to take appropriate action. During the autumn of 1948, UNTSO was re-established with an increase in size to supervise the Second Truce. The first group of observers to serve in Palestine under the UN Mediator, Count Folke Bernadotte, arrived in Rhodes at 6 P.M. GMT 20 July. It included 41 Americans and about 25 Belgians and were deployed on 21 July 1948. The initial group was quickly expanded to 93 in total because of the tremendous area that had to be covered. As the number of personnel grew, the United Nations Secretariat (of Personnel) supported the creation of the United Nations Truce Supervision Organization (UNTSO), the same organization UN Military Observers are assigned to today. Initially, the command was headed by a Chief-of-Staff (a general officer from one of the participating countries) in accordance with the personal direction of the Mediator, (a civilian).
Output (English): How many days after the observers left was the second truce called?