Essentially, the United Kingdom, France and the Soviet Union all allowed Germany to pursue its expansionist goals. They believed that giving concessions to Adolph Hitler was better than trying to oppose him at that point.
In the case of the United Kingdom and France, their leaders signed the Munich Pact with Hitler in 1938, allowing him to annex the Sudentland in Czech territory as part of Germany. Before long Hitler went on to take control of all of Czechoslovakia and later would invade Poland, and the British and French found themselves going to war with Germany after all.
In the case of the Soviet Union, the USSR and Germany signed a non-aggression pact in 1939, promising they wouldn't go to war against each other. That essentially gave Hitler permission to invade Poland without worrying about Soviet opposition. But by 1941, Hitler was breaking his promise and pursued an invasion of the Soviet Union.