The Battle of Sokolovo began on the Eastern Front, the same being an Allied delaying action near Kharkiv.
We may say "Allied" here, as it is regarded as the first instance of a "foreign" military unit serving alongside the Red Army, with that being the 1st Czechoslovak Independent Field Battalion. Of course, if we credit the fact that a lot of Red Army units were regional in nature, and those regions part of the Russian dominated USSR by force, it muddies the waters a bit, but perhaps not too much.
The unit's history is complicated in that the unit included Czech refugees from the Third Reich, but also Ukrainian Czechs who had been in Ukraine since the turn of the prior century. Relocating in western Ukraine, they were severely repressed by the Soviet Union. Following World War Two, most of them relocated to Czechoslovakia under a Czech law of return. Indeed, so many Ukrainian Czech joined the unit that they became the corps of the post-war Czech Army that existed in the newly formed Czechoslovakia.
