And today, in 1918, the Imperial German Army attacked . . .

the Polish II Corps in the Ukraine.

This particular Eastern battle after the peace between Russia, or rather the soon to be Soviet Union, and Germany is really confusing and perhaps the German action is somewhat understandable, although it once again demonstrates how the Germans managed to keep on fighting in the East after the Russians. .. or rather the Soviets, had surrendered.

Poland was mostly a Russian province prior to World War One but parts of it were in Germany.  During the war both the Central Powers and Imperial Russia formed Polish units, the Imperial Russian ones mostly by default but some with the goal of achieving some sort of Polish independence.  The Central Powers formed them with the same ostensible purpose, a safer bet for them as Polands vague borders put most of it in the East.  One of those units was the Polish Legions, which was a unit in the Austro Hungarian Army.

The II Brigade of the Polish Legion, lead by Polish patriot Jozef Haller von Hallenburg, had defected across the front lines to join the Russian Polish II Corps.  Haller, an eclectic Pole who had served before the war as an Austrian officer, viewed the terms of the Brest Litovsk Treaty as creating poor chances for Poland and lead that effort.  The Germans, for their part, viewed the Polish II Corps as a violation of the treaty and a threat to the Germans, which may have been a correct view.  On the night of May 10 they attacked the unit achieving a complete surprise but without the effect they hoped for.  Their casualties were heavy in the day long battle.  They did achieve a Polish surrender but only after a bloody fight.

Haller went on to become a Polish hero, surviving the battle and fleeing first to Moscow and subsequently ending up as a major campaigner for recruits for the Poles to serve on the Western Front.  He lead the Polish Blue Army in France.  

He was a significant military figure in post war Poland, commanding troops in the Polish war on its eastern borders that occurred in 1919 and again in the Russo Polish War (peace didn't really come to Poland in 1918).  He became a politician following the Russo Polish War and was living abroad when World War Two started, during which he became part of the Polish Government in Exile.  He did not return to Poland following World War Two and died in old age in London in 1960.