It all started when Japan's new Prime Minister, Sanae Takaichi, suggested that if China attacked Taiwan then Japan could respond with its own self-defence force. Since then, foreign ministries on both sides have lodged serious protests with the other, and a Chinese diplomat made what some interpreted as a threat to behead Takaichi. The spat touches on the historical animosity between China and Japan, as well as longstanding "strategic ambiguity" on the sovereignty of Taiwan.
Here's what to know.
