The United Kingdom, France, Italy, and Nazi Germany signed the Munich treaty to force Czechoslovakia to cede the mostly German-populated Sudetenland, a region that held the Czechoslovak border fortifications and defense line against German invasion, allowing Germany to invade and occupy Czechoslovakia a year later.
In this case, although morally objectionable, no previous promises from Britain to Czechoslovakia were broken.
On the other side, a mutual military assistance alliance between the United Kingdom and Poland was formalized by the Anglo-Polish Agreement in 1939. Upon the invasion of Poland by Nazi Germany, Britain declared war on Germany but did not launch a full land attack on Germany: most British bomber activities was the dropping of propaganda leaflets and reconnaissance. Without the promised British aid, Poland was overcome.
