Short answers, pulled from the story.
Trans-Olza is a territory in what is now the Czech Republic, situated in Cieszyn Silesia. Its name comes from the Olza River, which marks its western boundary with Poland. In Polish, the region is called Zaolzie, meaning "lands beyond the Olza," a term coined in the 1930s by writer Paweł Hulka-Laskowski.
Trans-Olza was disputed because 69.2% of its population identified as Polish in the 1910 Austrian census, giving Poland a strong ethnic claim to the territory. Czechoslovakia countered with economic and strategic arguments: the region held rich coal deposits and the Košice-Bohumín Railway, one of only two railroads linking the Czech provinces to Slovakia.
On the 28th of July 1920, the Spa Conference awarded Czechoslovakia 58.1% of the area of Cieszyn Silesia, containing 67.9% of the total population. The settlement left 139,000 Poles inside Czech territory and only 2,000 Czechs on the Polish side. Polish Prime Minister Wincenty Witos called it a decision that "has caused a rift between these two nations."
Poland held Trans-Olza for only 11 months. General Władysław Bortnowski led the Polish Army into the region on the 1st of October 1938, annexing 801.5 square kilometres with a population of 227,399. Nazi Germany invaded Poland on the 1st of September 1939, ending Polish control of the territory.
The death toll in Trans-Olza during World War II is estimated at about 6,000: approximately 2,500 Jews, around 2,000 other civilians of whom roughly 80% were Polish, and more than 1,000 locals conscripted into the Wehrmacht after being compelled to sign the Volksliste. At about 2.6% of its total population, Trans-Olza suffered the worst proportional human losses of any part of Czechoslovakia.
Polish president Lech Kaczyński publicly acknowledged it in 2009, at the 70th anniversary of the start of World War II. He stated that Poland's participation in the annexation of Czechoslovakia was "not only an error, but above all a sin." Czech and Slovak diplomatic delegations welcomed the statement.