The British commanders adopted a policy of avoiding reverses, before attempting to defeat the Senussi. Sollum was 280 mi  from Alexandria, too far west for a base and too exposed to German submarines, with the lack of fast patrol boats to guard ships in the bay. Mersa Matruh  was 120 mi  closer to Alexandria and had a good water supply. The Western Frontier posts were ordered back to Matruh to concentrate and to be reinforced by troops moved along the coast by trawler and on the Khedival Railway as far as Dabaa, 75 mi  short of Matruh. Orders were given on 20 November to form a Western Frontier Force, made up of composite horse and infantry brigades and supporting arms; by the end of the year, the British had about 40,000 troops in the Western Desert. On 21 November, the 2nd Battalion New Zealand Rifle Brigade, a company of the 15th Sikhs, parties of the Bikanir Camel Corps and an armoured train crewed by Egyptian gunners, was sent to Dabaa to guard the railway and patrol to the Moghara Oasis. Later on, the 1/1st North Midland Mounted Brigade was sent to Faiyum and a smaller force went to garrison Wadi Natrun, 45 mi  south of Alexandria.
Question: What endangered Sollum?
Answer: exposed to German submarines | lack of fast patrol boat