Why a tiny river on Russia’s border tests China’s ties with Kim Jong-un

The Tumen River has resurfaced as one of the big issues for keen observers of relations between China and North Korea but after the leaders of the two countries met this week, there was no mention of it in the official statements after the summit.
The river is a natural border between China, North Korea and Russia, and a narrow strip of it that runs between North Korea and Russia blocks Chinese access to open waters.
Beijing has long tried to convince its two neighbours to open the waterway to...