Xi's motivation to act during the "window of opportunity" provided by the US becoming an unreliable ally under Trump is possibly stronger than the deterrent provided by China's massive military expansion being not quite ready yet. China has been ramping up military "exercises" in the seas surrounding Taiwan, and are pushing things pretty hard with Taiwan's biggest regional ally, Japan. The coming year will be critical, as Xi might view the window as closed if the GOP gets crushed in the midterms.

Only if Trump's deranged rhetoric regarding Greenland actually results in an attempt to seize the island might a large-scale war be more likely. Denmark would immediately invoke NATO's Article 5, which considers any attack on a NATO member to be an attack on all members. The rest of NATO (besides the US) would either fight back or NATO would effectively cease to exist. NATO probably can't win that one militarily, but the resulting isolation and economic sanctions against the US would destroy the American economy and cripple the entire global economy even quicker than war with China would (although with the US heavily occupied in Greenland, I suspect Xi would immediately pounce anyway).