The writer is a former senior colonel in the People’s Liberation Army and now senior fellow at the Center for International Security and Strategy at Tsinghua University and China Forum expert

Will Putin use nuclear weapons in Ukraine? This billion-dollar question matters not only to Kyiv and Europe, but also to China. So far Beijing has trodden a careful line between Russia, its strategic partner, and Ukraine, which is a significant trading partner. During September’s Samarkand summit, Vladimir Putin thanked China for its “balanced position” on the Ukraine conflict.

But if Moscow decides to use tactical nuclear weapons against Ukraine, China can hardly maintain such a position any more. A joint declaration between Beijing and Kyiv in December 2013 agreed that China will not use or threaten to use nuclear weapons against Ukraine and, more importantly, will provide security assurances in the event of any such threat by a third party.