the british expected that berbera would be the objective of an italian invasion; the frontier with ethiopia was too long to guard and there was no position from whence to contest the approaches to the port through zeila near french somaliland, thence east along the coast road or through [[hargeisa]] or via [[burao]]. the inland mountains were passable by wheeled and tracked vehicles only on the hargeisa and burao roads, hargeisa being the more direct route through a gap at tug argan and the burao road through a defile known as sheikh pass. once north of the hills, the coastal plain had no feature where a small force could prevent the advance of a larger one. chater garrisoned tug argan with two battalions and the mountain artillery, one battalion to guard the other two approaches and kept one battalion in reserve. when the black watch arrived, it went into reserve and the 3/15th punjab regiment reinforced tug argan. the camel corps formed a screen in advance of the main defences to observe and delay an invader, with patrols of [[somali police force|illalo]] an armed constabulary, usually used as a frontier police force.