In early 1940 the Allies approved a naval campaign designed to seize the northern part of Norway, a neutral country, including the key port of Narvik, and possibly also to seize the iron mines at Gällivare in northern Sweden, from which Germany obtained much of its iron ore. As the Baltic froze in winter, the iron ore was then sent south by ship from Narvik. The Allies planned to begin by mining Norwegian waters, thus provoking a German reaction in Norway, and then would occupy much of the country. Unforeseen by the Allies, Germany had also planned to occupy Norway, and on 9 April German troops occupied Denmark and began an invasion of Norway. German forces quickly overran much of the country. The Allies sent troops to Norway, but they met with little success, and on 26 April the War Cabinet ordered a withdrawal. The Prime Minister's opponents decided to turn the adjournment debate for the Whitsun recess into a challenge to Chamberlain, who soon heard about the plan. After initial anger, Chamberlain determined to fight.