The United States sees the trade dispute it has with China as one of many. It's been down this road before, with Japan, Canada, Mexico and the European Union.
China views it differently. In Beijing, the trade issue is seen as a threat to its further development and a possible cause of instability. The compact between the party and the people, growth without political reform, could be damaged.
So, when the phrase, "don't say we didn't warn you" was used by state media last month it confirmed the gravity with which the situation is viewed from Beijing. This was not a throw-away line, though on the surface it seemed quite mild. That is until you realize the significance of it.