At that time Greece was theoretically neutral but in practice supported the British. The Italians expected the Greek army to collapse and the country to fall in a matter of days.
Unfortunately for them this did not happen. The Greeks managed not only to repulse the invasion but threw the Italians back into Albanian soil. Italy’s poor performance eventually forced Hitler to intervene and German troops invaded from the North and defeated the brave Greek warriors.
What was the secret of the unexpected initial Greek successes? Could it be that the Greeks were able to read the secret Italian codes? This is actually claimed by Wilhelm Flicke a member of OKW/Chi (Oberkommando der Wehrmacht/Chiffrier Abteilung) - Signal Intelligence Agency of the Supreme Command of the Armed Forces.
From ‘War Secrets in the Ether’ by Wilhelm Flicke, p138-9
Mussolini had decided on war in the Balkans. Von Papen's warnings made Hitler averse to any immediate action there, but he was only able to restrain Mussolini to the extent of limiting Italy to war with Greece. In less than two months the Italians, who had the advantage in everything save morale, were badly beaten. The political leaders were terribly surprised and the Chief of General Staff, Marshal Badoglio, and numerous other high officers were relieved of their duties. This did not help matters.
One of the most decisive factors during those weeks was the manner in which the Italians employed radio. The set-up was the same as that used in maneuvers of previous years. They employed open circular traffic; that is, they used one uniform frequency for a group of stations belonging to the same unit (e.g., the stations of three infantry regiments of a division for traffic with one another and with the divisional station) and each station used only one call sign for all its traffic. The call sign was supposed to change daily but was often used for several days; not infrequently a change in call sign was followed by errors which betrayed the change. Traffic was so heavy that the enemy always had a chance to take bearings and fix locations. Frequently messages were sent in clear. Several units of the Italian Eleventh Army distinguished themselves in this respect. Moreover, the Greeks had obtained at least two Italian army cryptographic systems, how I do not know, but it is certain that in the very first days of the campaign they could decipher a large part of the Italian messages. This enabled them to learn promptly most of the dispositions of the Italian command and to take appropriate action. The superiority thus gained was utilized cleverly and a series of military actions took place which heretofore would never have been deemed possible.
This is the first time I hear of such a success. No history book that I know of mentions this.