U.S. stock markets fell sharply after trading started on July 8 because investors became worried about fresh fighting between the United States and Iran. The biggest shock was in the Dow Jones Industrial Average, which fell 811 points, or 1.5%, shortly after the opening bell.Dow Jones fell nearly 800 points as Trump declared the Iran ceasefire over (Photo by Michael M. Santiago / GETTY IMAGES NORTH AMERICA / Getty Images via AFP) (Getty Images via AFP)The selling was not limited to the Dow. The S&P 500 fell around 1% , while the Nasdaq Composite was down about 1%. Investors became nervous after President Donald Trump suggested that the Iran ceasefire was likely over. Speaking at the NATO summit in Ankara, Turkey, Trump said, "I think it's over. I don't want to deal with them anymore. They're scum," according to CNBC.Trump warns of more strikesLater, Trump gave even stronger comments, saying the U.S. could launch more military action. He told reporters, "We're going to hit them hard tonight," according to Quartz. Trump's remarks came after the U.S. said it carried out a "series of powerful strikes" against Iran on Tuesday, as noted by CNBC.The U.S. strikes were launched after three commercial ships were attacked in the Strait of Hormuz. The Strait of Hormuz is one of the world's most important oil shipping routes, so any conflict there makes investors worry about global oil supplies. Because of these fears, oil prices jumped sharply.Oil prices jump higherBrent crude, the global oil benchmark, climbed as much as 8% to $80.07 per barrel in early trading. Later in the day, Brent crude rose even more, settling around $79.65 per barrel after gaining 7.5%. The U.S. oil benchmark, West Texas Intermediate (WTI), also surged 7.6% to $75.77 per barrel, according to Quartz.Also read: Why oil prices are surging? Impact on stock markets explained as Trump says Iran ceasefire is ‘over’Higher oil prices usually help energy companies because they can earn more money by selling oil. As a result, energy stocks moved higher while most other sectors struggled. Among energy companies, Marathon Petroleum gained about 4%. Shares of ConocoPhillips and Chevron each rose around 2%.Energy stocks gainAt the same time, companies that depend on lower fuel costs came under pressure because expensive oil can increase business expenses. Travel company Booking Holdings fell about 4%. Home improvement retailer Home Depot dropped around 3%. Fast-food giant McDonald's slipped by more than 1%.The market losses were broad-based. The Dow fell 811 points (1.5%), while both the S&P 500 and the Nasdaq Composite dropped 1%. Semiconductor stocks recovered slightly after falling the previous day. The VanEck Semiconductor ETF gained about 0.6%. However, the semiconductor ETF was still around 12% below its recent peak, showing the sector has not fully recovered.Fed meeting in focusAt the NATO summit, Mark Rutte defended the U.S. strikes, calling them "absolutely necessary". Rutte said the U.S. needed to respond because Iran had violated the ceasefire and commercial ships had been attacked, as cited by CNBC.Apart from the Middle East tensions, investors were also waiting for the minutes from the U.S. Federal Reserve's June meeting. The Fed minutes were expected to give traders more clues about future interest rate decisions and how policymakers are dealing with inflationOverall, the sharp fall in the Dow happened because investors reacted to renewed U.S.-Iran tensions, fears of more military action, rising oil prices, and uncertainty about the economy and interest rates.