Short answers, pulled from the story.
The history of Trans-Olza began in 1918 when Czechoslovakia claimed the area after the collapse of Austria-Hungary. The region was first mentioned in a written document on the 23rd of April 1155 when Cieszyn appeared in a letter from Pope Adrian IV.
Czechoslovakia received 58.1% of the area of Cieszyn Silesia following the Spa Conference decision on the 28th of July 1920. This territory contained 67.9% of the population and became known as Zaolzie from the Polish standpoint.
Nazi Germany unilaterally annexed Trans-Olza on the 26th of October 1939 as part of Landkreis Teschen. During the war, strong Germanization policies were introduced including mass killings such as the Żywocice tragedy on the 6th of August 1944.
The Košice Bohumín Railway was one of only two railroads that linked Czech provinces to Slovakia at that time. The area was crucial for Czechs because this railway line connected Czech Silesia with Slovakia.
Poland annexed an area of 801.5 km2 with a population of 227,399 people starting on the 1st of October 1938. Trans-Olza remained a part of Poland for only 11 months before being returned to Czechoslovakia immediately after World War II.