On the back in German:
Schz Josef Köhler, Gef. am 18. Sept. 1939 in Janow.
PVT Josef Köhler, fallen September 18, 1939, in Janow.
The battle of Jarowow and Janow Forests took place on the 14 - 16th of September, so this is possibly a soldier defending reinforcements from central Poland trying to reach Lwow (Lviv, Ukraine) to reinforce units there for the Battle of Lwow.
An extremely rare photograph from the very first days of the war.