 Wilson faced the prospect of being pinned by Germans operating from Thessaloniki , while being flanked by the German XL Panzer Corps descending through the Monastir Gap . On 13 April , he withdrew all British forces to the Haliacmon river and then to the narrow pass at Thermopylae . On 14 April , the 9th Panzer Division established a bridgehead across the Haliacmon river , but an attempt to advance beyond this point was stopped by intense Allied fire . This defence had three main components : the Platamon tunnel area between Olympus and the sea , the Olympus pass itself and the Servia pass to the south @-@ east . By channeling the attack through these three defiles , the new line offered far greater defensive strength . The defences of the Olympus and Servia passes consisted of the 4th New Zealand Brigade , 5th New Zealand Brigade and the 16th Australian Brigade . For the next three days , the advance of the 9th Panzer Division was stalled in front of these resolutely held positions . 
