Bernie Sanders often points to Europe as his economic model, but there’s one lesson from the Continent that he and Elizabeth Warren want to ignore. Europe has tried and mostly rejected the wealth taxes that the two presidential candidates are now promising for America.

Senator Warren’s plan sets an annual 2% tax on assets above $50 million, and the rate rises to 6% for billionaires. Bernie Sanders wants to tax joint filers’ wealth between $32 million and $10 billion at rates of 1% to 8%. His advisers brag that this could wipe...