"The Invasion of Slavonia"
Major Thorpe (writer and artist), Igor Shop (pencils and inks)
Published in Jumbo Comics #25, Mar 1941 - 5 pages
ZX-5 receives an urgent telegram requesting his help from an old friend named Jan Petrov, mayor of a town in Slavonia, a tiny neutral country in the Balkans being "courted by each of the warring powers". He heads there immediately but finds only a deserted city, his friend dead from apparent suicide, and an invading army heading into town.
Here are the pages I have of it.