Discussions over the possible German invasion of Ireland had been ongoing in Britain since the beginning of 1939. In June 1940, Britain's political and military establishment had witnessed the seemingly invincible German Blitzkrieg which led to the defeat of Poland, the Low Countries, and France, and the retreat of the British Expeditionary Force from Dunkirk. The British suspected that, following their defeat in France, the next step would be a German invasion of Britain – Operation Sea Lion. They did not know, but also suspected, that there was a plan to invade neutral Ireland – Operation Green.