It's widely known that Hitler and Stalin dismembered Poland in 1939. Little known is that, a year earlier, Poland had joined Hitler in dismembering Czechoslovakia.
This ironic bit of historical trivia appears in Volume One of The Illustrated World War II Encyclopedia. Hardly a work of conspiracist revisionism [see its credits at the bottom of this article], its recounting of Hitler's dismemberment of Czechoslovakia is well known, apart from the details concerning Hungary and Poland’s roles.
According to the Encyclopedia, in May 1938 Hitler mobilized his military to annex Czechoslovakia's German-speaking Sudetenland. When Britain, France, and the USSR threatened war, Hitler backed down, but continued pressing the issue. On September 15, Britain's Neville Chamberlain visited Hitler to discuss a peaceful solution. On September 22, Chamberlain agreed to allow Hitler to annex the Sudetenland but refused to permit immediate entry for German troops, thus Hitler remained dissatisfied. On September 23, Czechoslovakia mobilized its military and war looked imminent.
Then Poland made its move. On September 27, seeing Czechoslovakia in crisis as Germany prepared to invade, Poland issued an ultimatum demanding that Czechoslovakia cede its Tesin (Teschen) district.
On September 29, France, Britain, Germany, and Italy signed the Munich Agreement. This allowed Hitler to take the Sudetenland in exchange for him agreeing to “guarantee” Czechoslovakia's borders — but only after Poland and Hungary (which by now had joined in) had taken their shares.
The Encyclopedia reports: “As Article 1 of the [Munich] agreement put it, ‘when the question of the Polish and Hungarian minorities in Czechoslovakia has been settled, Germany and Italy will each give a similar guarantee to Czechoslovakia’. Poland had been first to share in the spoils. After an ultimatum from Warsaw on September 27, 1938, Czechoslovakia had ceded to Poland the district of Tesin (Teschen) — an area of some 625 square miles with a population of 230,000 people.”