Most Western accounts of the Second World War date its start to 1 September 1939, when Germany invaded Poland. That date reflects when the war began for the European powers whose entry defines the conflict's most familiar Western narrative. In much of East Asia, particularly China, the same broad global conflict is often dated instead from 7 July 1937, when a clash between Japanese and Chinese forces escalated into a full-scale, sustained war between Japan and China — a conflict that was already well underway, with enormous casualties, more than two years before the war most Western accounts treat as the Second World War's actual starting point.
Two wars that began separately and only gradually merged into one
The war in Europe, beginning with Germany's invasion of Poland, and the war in Asia, beginning with Japan's full-scale invasion of China, started as genuinely separate conflicts, driven by distinct regional causes and involving largely distinct sets of combatants in their earliest years. The two conflicts didn't formally merge into what's now conventionally treated as a single global war until later — most decisively after Japan's attack on Pearl Harbor in December 1941 brought the United States into both the Pacific war against Japan and, through Germany's subsequent declaration of war on the US, the European conflict as well, drawing what had been two largely separate regional wars into one interconnected global conflict with shared, allied combatants fighting on both fronts.
Which starting date a country uses reflects its own war experience
Which starting date a given country's historical memory and education system actually emphasises tends to track that country's own direct experience of the conflict: for countries whose war began with Germany's actions in Europe, 1939 marks a genuine starting point of their own war experience. For China, whose sustained, devastating war with Japan began in 1937 and continued essentially without pause through the war's eventual global merger and end, dating the war's true beginning to 1939 would sharply understate more than two years of severe conflict China had already experienced by that point. Neither convention is simply "wrong" in an absolute sense; each reflects a genuine, differently timed starting point for a specific set of combatants within what eventually became, after 1941, a single interconnected global conflict involving both regional theatres.
What we're still unsure about
The separate origins of the European and Asian theatres, and their gradual merger into a single global conflict following Pearl Harbor, are well documented and widely accepted by historians, drawing on extensive military and diplomatic records from all major combatants. What remains more a matter of ongoing historiographical and national perspective than a simple factual dispute is exactly how the war's overall periodisation should be framed in historical writing and education — different national historical traditions continue to emphasise different starting points and different theatres as most central to "the" war's story, a difference in framing and emphasis rather than a disagreement over the underlying documented sequence of events itself.
This sits inside World War II, one of eight topics in Modern History, one of seven domains in History, one of seventeen subjects the app can quiz you on.