For me, Japan's large-scale military attack in China in 1937 was more than a "political event".
It's not so clear-cut where policy ends and war starts, and when a war turns into a global war, but I cannot say that September 1, 1939, would clearly be where we draw the line. The fighting in China in 1937 (Battle of Shanghai) was actually bigger in scope than the invasion of Poland in September 1939, and with more casualties. Then the Winter War in Finland exceeded these, with big losses for USSR, and then large-scale war started on the Western front in the Battle of France in 1940 which was even bigger than Winter War in terms of casualties.
It's not so clear-cut where policy ends and war starts, and when a war turns into a global war, but I cannot say that September 1, 1939, would clearly be where we draw the line. The fighting in China in 1937 (Battle of Shanghai) was actually bigger in scope than the invasion of Poland in September 1939, and with more casualties. Then the Winter War in Finland exceeded these, with big losses for USSR, and then large-scale war started on the Western front in the Battle of France in 1940 which was even bigger than Winter War in terms of casualties.