After Poland was defeated, the "Phoney War" began in Western Europe. While British soldiers were sent to the Continent, there were no big battles fought between the two sides. Then, in April 1940, Germany decided to attack Norway and Denmark so that it would be safer to transport iron ore from Sweden. The British and French sent an army to disrupt the German occupation, but had to leave when Germany invaded France. Chamberlain was replaced by Churchill as Prime Minister of United Kingdom in May 1940 because the British were unhappy with his work.