Switzerland’s central bank triggered unprecedented turmoil in the currency market on Thursday after it unexpectedly scrapped a cap on the franc-euro rate, underscoring the difficulty central banks have protecting their economies from developments beyond their borders.

The abandonment of the cap, which had essentially pinned the currency at 1.20 francs a euro for the last 3½ years, prompted a 33% collapse in the euro versus the franc—the biggest single-day move in a developed market traders could recall. Swiss stocks fell nearly 10% as traders worried the stronger franc would hurt Switzerland’s exports, especially to Europe.

With Thursday’s move, Switzerland became the first domino to fall in anticipation of the European Central Bank’s decision on whether to purchase massive amounts of eurozone government bonds to raise the money supply and boost the currency area’s sagging economic prospects. Most analysts expect the ECB to launch such a policy, known as quantitative easing, at its Jan. 22 meeting.