Thursday’s ECB meeting is shaping up to be one of the most important in recent years. The bank’s president, Mario Draghi, put financial markets on high alert for June action four weeks ago when he said the ECB was “comfortable acting next time.”

Other officials have since struck a similar tone. Time is running out. Annual euro-zone inflation weakened last month to just 0.5%, far below the ECB's target of just below 2%. Grim economic reports have led to widespread expectations that the ECB will unveil a package of stimulus measures Thursday but stop short of large-scale asset purchases.