NEW YORK — With no clear end in sight, the war with Iran is sending oil prices back to $100 per barrel and stocks sinking worldwide on Thursday.

The S&P 500 fell 1.1% and is returning to big swings following a couple days of relative calm. The Dow Jones Industrial Average was down 588 points, or 1.2%, as of 10 a.m. Eastern time, and the Nasdaq composite was 1.4% lower.

The center of action was again the oil market, where the price of a barrel of Brent crude, the international standard, got as high as $101.59 overnight before pulling back to $99.50, an 8.1% rise. Worries remain high that the war could block the production and transport of oil in the Persian Gulf for a long time, which in turn could cause a debilitating surge of…