battalions each, and supporting troops. After the invasion of Poland in 1939, the old divisions were partially reorganised (adding a third battalion to some infantry regiments or alternatively adding a second regiment of two battalions). Around this time, the newly organised divisions (6th through 10th) diverged in organisation, each on average with one tank regiment, one separate tank battalion, one or two infantry regiments (three to four battalions per division). By the start of Operation Barbarossa, the German invasion of the Soviet Union in 1941, the 21 panzer divisions had undergone further reorganisation to now consist of one tank regiment (of two or three battalions) and two motorised regiments (of two battalions each). Until the winter of 1941/42, the organic component of these divisions consisted of a motorised artillery regiment (of one heavy and two light battalions) and the following battalions: reconnaissance, motorcycle, anti-tank, pioneer, field replacement, and communications. The number of tanks in the 1941-style divisions was relatively small, compared to their predecessors' composition. All other units in these formations were fully motorised (trucks, half-tracks, specialized combat vehicles) to match the speed of the tanks. During the winter of 1941/42, the divisions underwent another reorganisation, with a tank regiment comprising from one to three battalions, depending on location (generally three for Army Group South, one for Army Group Centre, other commands usually two battalions). Throughout 1942, the reconnaissance battalions were merged into the motorcycle battalions. By the summer of 1943, the Luftwaffe and Waffen-SS also had panzer divisions. A renewed standardization of the tank regiments was attempted. Each was now supposed to consist of two battalions, one with Panzer IV and one with Panther (Panzer V). In reality, the organization continued to vary from division to division. The first infantry battalion of the first infantry regiment of each panzer division was now supposed to be fully mechanised (mounted on armoured half-tracks (Sd.Kfz. 251). The first battalion of the artillery regiment replaced its former towed light howitzers with a mix of heavy and light self-propelled artillery (the Hummel with a 15 cm sFH 18/1 L/30 gun and the standard 105mm howitzer-equipped Wespe). The anti-tank battalion now included assault guns, tank destroyers (Panzerjaeger/Jadgpanzer), and towed anti-tank guns. Generally, the mechanization of these divisions increased compared to their previous organization. Since the Heer and the SS used their own ordinal systems, there were duplicate numbers (i.e. there was both a 9th Panzerdivision