The bond-market rally this year has taken most investors by surprise. But the truly remarkable outcomes are to be found in the euro zone.

In Germany, 10-year yields at 1.28% are close to the record lows of 2012, when the euro zone appeared on the brink of implosion. In France, 10-year yields have hit all-time lows based on data going back to before the revolution. In the Netherlands, yields are at their lowest levels in half a…