On this day, British forces in East Africa, acting on information obtained by breaking the Italians' coded messages, invade Italian-occupied Eritrea-a solid step towards victory in Africa.
British Intelligence had been privy to secret Italian communiques from Africa for the past five months; every instruction sent from one Italian military unit to another was analyzed by the Brits. The Italian viceroy in Ethiopia was unwittingly receiving and transmitting every Italian military secret-and weakness. Consequently, British forces were able to organize a strategy to advance on Italian-occupied territory, with Italian troop movements in mind.
On January 19, news of an Italian withdrawal from the town of Kassala, in Anglo-Egyptian Sudan, which the Italians had occupied since July 1940, reached British ears. The British garrison there had been slow to react initially to the Italian invasion of Sudan, preferring to wait to get a clearer picture of the Italian invasion strategy for East Africa. The British bided their time by beefing up their forces, especially tank forces, to something closer to parity with the Italians'. The Italian withdrawal from Kassala, a proactive defensive movement, provided the perfect opportunity for Gen. William Platt and the Indian divisions to launch an assault on Eritrea, which bordered Sudan and Ethiopia. It was not long before Italian-occupied Ethiopia and Somaliland fell.
Taken from: http://www.history.com/this-day-in-history/british-attack-italians-in-africa [19.01.2015]