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 channelling 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.