The attack was a two horn type, the start of Blitzgrieg, the Nazis from one side and the Russians from the other, that photo is one of many where the demarcation line was decided between the two invaders, interesting is that they never collaborated in any joined attack together though this was decided that the attack started by the German and the retreating Polish Army was then completly crashed by the Soviet pincer movement. One has to understand that by that time Soviets had already one of the top tank army infects tanks were already better then the Germans [tested against each other at the Spanish Civil war]. This action led also to the massacre of the top Polish officers by the Soviets.
Iv