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.

At the same time as the developments in the Klidi Pass, Major General Gifard McKay ordered the withdrawal of the Greek detachment from the positions it had 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 Vlasti, 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 led by Colonel Sergios arrived at Amyntaio as planned and proceeded to Ptolemaida. However, the convoy of Lieutenant Colonel Nikolaou ran into problems. One squadron was diverted to support the Australian forces fighting at Klidi. This squadron surrendered to the Germans after suffering losses of ten men. The rest of the squadron arrived at Amyntaio, where they came under heavy fire from German tanks and artillery. With the intervention of British tanks, the German tanks were pinned down, allowing the Greek forces to escape without casualties.

As for the left part of the defense line, which extended to the west of the Klidi Pass, a strong German attack took place there at about 16:00 on 12 April. The Greek 1st Battalion of the 88th Infantry Regiment, which had taken over the defense of the left section and was positioned south of the village of Flamburo, was hit by a barrage of artillery and mortar fire. At the same time, two German battalions advanced on the defensive lines and at about 18:30 attacked the Greek forces, causing them to retreat. Some of the men of the 1st Battalion surrendered to the Germans or were captured, while others retreated disorderly to the south and through the woods toward the village of Aetos. The commander of the 88th, Reserve Lieutenant Colonel George Chondros, attempted a counterattack with a small reserve force. Revolver in hand, he charged a German section but was killed by automatic rifle fire.

During the battle, the Greek infantry battalion suffered heavy losses: 11 dead, including one officer, 18 wounded, and 96 prisoners, including 6 officers. The Germans, with equally heavy losses, occupied the heights north of Aetos by evening.

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.