 Complete the following phrase:  In May 1915, the 13th Light Horse Regiment left Australia for the Middle East, arriving in Egypt in June. When the Australian infantry units had been dispatched to Gallipoli earlier that year, it was thought the terrain was unsuitable for mounted troops, and the light horse regiments had remained in Egypt. However, heavy casualties resulted in their deployment in May and the 13th Light Horse Regiment in turn was sent into battle in September 1915 in a dismounted role. The regiment was deployed defending the trenches at 