On April 8, the troops of the "W" group - the coalition of Greek, British and Australian troops - set up a defense against the Germans on the line of Kelli - Klidi Pass - Nymfaio area line. At the head of the group was the commander of the 6th Australian Division, Major General Gifard McKay. At the right end of the defense, south of Kelli, was a Greek detachment consisting of troops from the 20th Division, the Dodecanese Regiment and the X Border Sector. At the head of the detachment was the reserve permanent infantry colonel Aristotelis Sergios. In the center, at the Klidi Pass, the defense of the area was undertaken by Australian and British forces. On the left, the maintenance of the Greek territory was entrusted to the 21st Greek Brigade.

On the night of April 9-10, the constant snow and cold challenged the Allied troops' ability to operate, especially the Australians, who were unfamiliar with such weather conditions. At the same time, the news from Central Macedonia was discouraging: despite heroic resistance at the forts on the Metaxa Line, the Germans advanced to Thessaloniki, and the Eastern Macedonia Army Section surrendered. The situation was no better in the northwest. On April 10, the Germans captured Florina and began preparations for their advance to the Klidi Pass. At night, taking advantage of the darkness and weather conditions, they conducted limited reconnaissance operations and captured eleven Allied soldiers.

On April 11, strong German forces advanced into the Klidi Pass. British and Australian units of "W" Group tried to repel them with artillery and antitank fire, and coordinated attacks with the help of the reconnaissance battalion. At the same time, severe cold, snowfall, and the laying of mines immobilized the tanks of the German XL Armored Corps. Although the Allied forces were unable to stop the German advance, they were able to slow its movement southward.

On April 12, on the front in the Klidi Pass, the Germans continued to advance south at a slow pace, despite the heavy losses suffered by the Allied artillery. Increasing pressure on the Allied battalions caused the British to retreat toward Amyntaio and the Australians toward Sotiras.

Simultaneously with the developments in the Klidi Pass, Major General Gifard McKay ordered the withdrawal of the Greek detachment from the positions occupied south of Kelli and their concentration in Amyntaio. From there, part of them were to be moved by British vehicles to Ptolemaida and then to Vlastis, while the rest were to move from Amyntaio to Klisoura. The withdrawal took place in two convoys: the first, led by Colonel Aristotle Sergios, went east of Lake Petra, and the second, led by Lieutenant Colonel Ioannis Nikolaou, crossed the western side of the lake.

The convoy of Colonel Sergios arrived in Amyntaio according to the program and from there it went to Ptolemaida. On the contrary, the convoy of Lieutenant Colonel Nikolaou encountered problems. A squad of the phalanx deviated from the planned route and went to the village of Klidi to support the Australian battalion fighting there. This action resulted in the deaths of ten soldiers and the eventual encirclement and surrender of the squadron to German forces. The remainder of the convoy managed to reach the west of Amyntaio at about 18:00, but as it approached the area it came under flanking fire from German tanks moving from the southern exit of the Klidi Pass and parallel artillery fire from the northwestern positions of the pass. With the intervention of 25-30 British tanks, the German tanks were intercepted, and the Greek convoy was able to escape towards Amyntaio, avoiding capture.

Finally, the German forces occupied the area of the Klidi Pass after three days of continuous fighting, despite the fierce resistance of the "W" group. The resistance of the Allied forces caused significant losses in the German ranks, both in terms of human lives and combat material. Nevertheless, the fall of the pass paved the way for the further advance of the German troops into the territories of Western Macedonia and eventually into Central and Southern Greece.