Why had international peace collapsed by 1939?

Timeframe drift — the question ends at 1 September 1939 — 'Why had international peace collapsed by 1939?' covers events up to the German invasion of Poland. Events from WWII itself (Dunkirk, the Battle of Britain, Operation Barbarossa, the Holocaust) are completely outside scope and will not be credited. Also avoid drifting backwards into WWI causes — the focus is the 1930s crisis years. Every piece of evidence you use must fall within the question's date range.

Support every claim about appeasement with named evidence — writing 'Chamberlain was wrong to appease Hitler' earns Level 2 without specific cases. You must cite named events: the Rhineland remilitarisation (March 1936, no Allied response), Anschluss (March 1938, no action), the Munich Agreement (September 1938, Sudetenland surrendered), and Prague (March 1939, when Chamberlain finally abandoned appeasement). You should also present the genuine case FOR appeasement — public hostility to another war, the need for rearmament time, economic constraints, and genuine uncertainty about Hitler's ultimate aims.

Avoid single-factor explanations — attributing the collapse of peace solely to Hitler's aims, or solely to appeasement, limits answers to Level 3 at best. A Level 5 answer identifies and explains multiple factors — Hitler's revisionist foreign policy, the failure of collective security, the Depression's destabilising effect, appeasement, and the Nazi–Soviet Pact (August 1939) — and reaches a supported judgement about which factor was most significant.