First a quick recap: there was $6.3 trillion in US Treasuries held by foreign nations as of Dec. 2017, of which over $4 trillion was held by official accounts: central banks, reserve managers, sovereign wealth funds, and others.

Also recall that much if not all of these official foreign Treasury holdings built up over the years as US trading partners converted dollars from persistent American trade surpluses into US debt.

Which is why, as Reuters' Richard Leong writes, should China, Japan and other nations, which have recycled their trade dollars through their Treasuries holdings, suddenly decide to whittle them down, "markets could be in for a rough ride."