By late 1938 it was clear that war was looming, and that Germany had the world's most powerful military. British military leaders warned that Germany would win a war, and Britain needed another year or two to catch up in terms of aviation and air defence. Appeasement of Germany—giving in to its demands—was the government's policy until early 1939. The final act of appeasement came when Britain and France sacrificed Czechoslovakia to Hitler's demands at the Munich Agreement of 1938. Instead of satiation Hitler then seized the rest of Czechoslovakia in March 1939 and menaced Poland. In response Prime Minister Neville Chamberlain rejected further appeasement and stood firm in promising to defend Poland. Hitler unexpectedly cut a deal with Joseph Stalin to divide Eastern Europe; when Germany did invade Poland in September 1939, Britain and France declared war; the British Commonwealth followed London's lead.