Historians of appeasement make different assumptions about Britain’s ability to influence events through the threat or use of military force or economic sanctions. Attempts at measuring power are examined critically and key factors identified, including the armed forces, the arms industry, the wider economy and public willingness to support foreign policy. The strength of the Royal Navy is discussed in relation to the size of other navies and to Britain’s commitments, and Churchill’s comment that the Chamberlain government did not neglect the navy is noted. In contrast, Churchill described the loss of Britain’s lead over Germany in air power as a disaster. Britain and Germany’s air power are compared in the context of technical developments. Britain’s limited capability to support France on land is explained, with particular reference to tanks, which Churchill had pioneered. Historians’ debates about the adequacy of Britain’s arms industry are discussed in relation to problems encountered by Germany in rearmament. It is argued that the principal reason why Britain lagged behind Germany was Chamberlain’s unwillingness to accept Churchill’s advice to divert industry from civil trade and industry to producing munitions. The possibilities of collective security through the League of Nations or Churchill’s concept of a grand alliance are explored and the importance of intelligence in influencing perceptions of power emphasized.