I am going to say that the main purpose of a battle tank is to break through the enemies front line while at the same time destroying pillboxes, bunkers other tanks etc. so the infantry can move in and they can fire while on the move....

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The war against Russia will be such that it cannot be conducted in a knightly fashion. This struggle is one of ideologies and racial differences and will have to be conducted with unprecedented, unmerciful and unrelenting harshness. -Adolf Hitler