Following the collapse of resistance in Yugoslavia, the left flank of the Vermion line held by Greek and British Empire forces became exposed.

Leibstandarte SS Adolf Hitler (LSSAH), a brigade group-level unit commanded by Oberstgruppenführer Josef "Sepp" Dietrich and accompanied by 9th Panzer Division,[13] advanced further south and occupied the town of Vevi on 11 April.

Its task, in the words of the British Empire's commander in Greece, General H. Maitland Wilson, was to "stop a blitzkrieg down the Florina Valley.

These hindered the ability of the Allied infantry to sleep,[12][19] especially the Australians and New Zealanders – who were tired after a long and sudden journey from North Africa, and were not experienced in or equipped for wintry, mountain conditions.

Vasey's headquarters were located a substantial distance away, approximately 10 kilometres (6.2 mi) south of the town of Klidi.

Apell would move (after the LSSAH attack) from the area of Flambouron toward Aetos and Xino Nero (Eksisou), and the rear of the forces defending the pass.

[25][32] On the Dodecanese Regiment's front at midnight (10/11 April), the Germans attempted reconnaissance patrols and probing attacks against Glava Hill, but were repelled after a two-hour battle.

My orders flew over the wire and the first rounds screamed through the air... A few furious moments and back went the Hun [a derogatory term for Germans], but five trucks stayed in the road as silent witness that my troop could shoot".

In the words of the Australian War Memorial: "[d]espite being strung out and exhausted from a long march to the position and bitterly cold weather, the 2/8th managed to fend them off.

This opened the pass itself to the Germans, created a gap between the 2/4th and 2/8th Battalions, severed communications between Vasey and the 2/8th and left Australian anti-tank guns without infantry protection.

[33][36] However, in the words of Australian official historian Gavin Long: "At 2:00 pm ... Lt.-Col. [John] Mitchell of the 2/8th ... ordered a counter-attack which regained some vital ground on top of the ridge... After six hours of intermittent fighting in the pass and on the slopes to the east, the 2/8th still held the Hills though their left had been mauled; the Rangers, however, were rallying astride the road about two miles to the rear, but five of the six supporting guns of the 2/1st Anti-Tank Regiment had been left without protection and abandoned.

[26] By 15:30, the 2nd and 3rd Companies had captured Hill 1009 in the 1st Rangers' sector, reducing British machine gun positions with heavy weapons.

KG Witt attacked forward, and pioneers coming behind the 3rd company started to open a corridor in the Allied minefield eventually allowing two assault guns (StuG) to pass through.

[26] With German artillery fire on the Greek lines becoming more intense, at 15:40, Mackay ordered the Dodecanese Regiment to retreat immediately and to have completed the evacuation of its positions by 18:00 (bringing forward the withdrawal scheduled for 19:00).

[26] In the west at 16:00, the German attack was extended against the Greek 1/88th Battalion, which began taking artillery and mortar fire.

The 2/8th had been exposed on two flanks by the Greek and British withdrawals; it was soon coming under German machine gun fire from the east.

At 17:00, he telephoned the commanding officer of the 2/4th Battalion ... with the code phrase indicating that a pull-out was now vital – "the roof is leaking.

"[12] At 17:30, the Australian official history reported that 500 German infantry supported by self-propelled guns attacked in force along the width of the 2/8th's sector.

[37] A German participant of the battle, Obersturmbannfuhrer Kurt Meyer, wrote later of his surprise at how easily the heavy StuGs had climbed the slopes.

The 2/8th Battalion was forced into a chaotic retreat, with component units being separated and officers ordering the abandonment even of light weapons, to speed the withdrawal.

Losses among the Australian infantry would have been much worse it were not for the 2/1st Anti-Tank Regiment and the British Royal Horse Artillery standing their ground in the centre, until the Germans were only 400 metres (440 yards) away.

Its right column (consisting of the III and I battalions, as well as the regimental HQ company) was attacked by "about 20 German tanks" (actually six StuG and nine PzJg I) at 18:00 west of Amyntaion, with fire from a distance of 800–1,000 m (870–1,090 yd).

The Dodecanese Regiment, the most combat-worthy of the 20th Division's units, was scattered during the westward withdrawal, due to misunderstandings with the British who undertook their transport on lorries as the regiment’s commander chose not to show up at the scheduled rendezvous with Battalion I and III, and remained as the reserve of the 20th Division for the remainder of the war.

Siniatsiko when the German LSSAH brigade made contact with the main defensive line at Kleisoura in the evening on 13 April.

Although the British 1st Armoured Brigade fought delaying battles at Sotir and Proastio during 13 April, covering 12th Division's withdrawal, the latter was eventually split by the German advance, losing three of its battalions which were cut off the main body and retreated south towards Servia.

[47] When the German 9th Panzer Division made contact with the Siatista defensive location at 13.30 on 14 April, only one battalion of the 82nd Regiment was in position.

[48] When expecting the German assault on Siatista pass on 15 April, 12th Division could muster only 1,000 men as an effective strength; it was greatly demoralised and fatigued.