It began in the early morning of 9 March 1941, when Fascist Italy launched an assault (Operation Primavera) against Greece, aimed at capturing the critical mountain pass leading into the Kalpaki valley.

Hill 731, strategically located 20 km north of Klisura (Këlcyrë) at the feet of Mount Trebeshinë, stood at the heart of the Greek defensive line.

The plan, devised by General Ugo Cavallero, envisioned a large-scale attack on a narrow, 32 km front in the centre of the Greek positions.

The commander of the Greek II Corps Maj. Gen. Georgios Bakos had been appointed a mere four days prior to the Italian offensive: during a conference held at Athens on 5 March, the then commander Lt. Gen. Dimitrios Papadopoulos, along with two other generals of the I Corps, objected to the presence in Greece of a British expeditionary force, which would have been inadequate to defend Greece against the Germans but sufficient to provide them with a casus belli.

Dimitrios Kaslas, after the bombardment gave an order for every soldier to stand his ground no matter what, and promised to personally shoot and kill anyone trying to retreat.

The Italian offensive halted between 16–18 March, allowing the Greeks to bring reserves forward and begin a gradual reshuffle of their line, relieving the 1st Division with the 17th.

Supported by four M13/40 tanks and an Arditi assault unit from the Siena Division, the Italians seized a portion of Hill 731 but were soon pushed back by a Greek counter-attack.

For several days Allied troops contained the German advance on the Thermopylae pass, allowing ships to be prepared to evacuate the British force.