Maybe the short conflict they had with Vietnam? Though it’s arguable whether they started it or Vietnam did. Vietnam did attack Cambodia which was a Chinese ally at the time. Imo Vietnam was justified to intervene in Cambodia because of what was happening there, but this does mean that legally China then going into Vietnam is not a clear cut case of who started it and could be construed as a retaliatory reaction. Overall the foreign policy of the PRC at that time was not great, the Sino-Soviet split led to China making some poor choices. But is nothing compared to what the US has done and all of the wars they started (and don’t forget that the Europeans were also deeply involved with helping the US in most of its wars). For being as powerful as it is, China is one of the most peaceful countries in the world.