Investing in emerging markets is like watching the tides without the moon as a guide. Capital flows in and out in surges, and woe to the country that gets caught with bad policies when the tide suddenly goes out. Argentina has been getting that re-education of late, and now Turkey is watching capital flee for safer climes.

Turkey’s lira fell as much as 5% on Wednesday, and it’s lost more than a fifth of its value this year, adding to a long decline that is forcing extreme monetary measures to compensate. Much of the blame...