After-Market Summary: Oil Surge and Higher Yields Weigh on August's Final Session

U.S. stocks closed lower on Monday, August 31, as renewed military action involving Iran pushed oil prices higher and revived inflation and interest-rate concerns. The decline ended an otherwise positive August for the three major large-cap indexes, while energy shares were a notable exception to broad sector weakness.

Market at a Glance

The selling was broad: nearly every S&P 500 sector finished lower. The Dow's larger percentage drop indicated particular pressure on its economically sensitive and industrial constituents, while the Nasdaq's relatively small decline reflected some resilience in large technology shares.

For August, the S&P 500 rose 2.6% and the Nasdaq gained 3.9%; the Dow posted its fifth consecutive monthly increase. Those monthly gains provide context for Monday's decline, but do not reduce the near-term risks created by higher energy prices and rising Treasury yields.

Strait of Hormuz Escalation Lifts Oil

U.S. forces struck Iranian rocket launchers near the Strait of Hormuz on Sunday, the first reported American military action in a month. The United Arab Emirates separately said it intercepted an Iranian drone over its waters on Monday.

The Strait of Hormuz carries roughly one-fifth of global oil shipments. Its disrupted traffic has been a central source of volatility in energy markets throughout the conflict. Brent crude rose 2.7% to settle at $90.49 per barrel, after falling below $80 earlier in August.

Higher oil prices can feed into gasoline, transportation, and goods costs, complicating an inflation outlook that was already above the Federal Reserve's 2% target. The direct cause of future inflation cannot be established from one trading day, but the move adds a clear upside risk if oil remains elevated.

Energy shares reflected the price move: Exxon Mobil gained 2.7% and Chevron rose 2.1%, while the broader market declined.

Treasury Yields Return to Recent Highs

The 10-year Treasury yield rose to 4.75% from 4.73% late Friday, returning to roughly the level seen two weeks earlier when the Treasury Department announced an unusual intervention in the bond market. The two-year yield held at 4.34%, still far above its level near the start of 2026.

The oil rise and higher long-term yields increased concern that the Fed may need to tighten policy further. Futures tracked by CME FedWatch placed the chance of a rate increase at the September meeting at 66%, according to the Associated Press. That is a market-implied probability, not a forecast of the Fed's eventual decision.

The next key inflation report is due September 11, just before the Federal Reserve's September 15-16 meeting. The August employment report later this week is the other important policy input; July payrolls unexpectedly fell by 23,000, with earlier months revised lower.

Company Movers: Utilities and Amazon Decline

Edison International fell 23.1% and PG&E dropped 20.1% after reports about potential California wildfire legislation that could permit insurers to sue utilities over related claims. The moves were company- and policy-specific rather than a signal about the entire utilities sector.

Amazon lost 2.5% following a Wall Street Journal report that the Federal Trade Commission and more than 20 states were preparing an antitrust suit alleging price manipulation on its platform. The potential legal action had not been filed at the time of the session, so investors should distinguish the report from a completed enforcement proceeding.

GameStop rose 2.9% after offering a preliminary second-quarter outlook above its year-earlier results. Aon declined 9.5% after announcing it would acquire USI Insurance Services from KKR in a transaction valued at $17 billion including debt.

What Investors Should Watch Next

Developments around the Strait of Hormuz are the most immediate macro risk. A sustained increase in oil prices could make upcoming inflation readings harder to improve and raise pressure on the Federal Reserve.

The August jobs report will help determine whether the Fed sees signs of weakening labor demand sufficient to offset inflation concerns. Markets will also monitor the 10-year yield: a sustained move above 4.75% could tighten financial conditions and challenge equity valuations.

Oil-sensitive sectors may continue to diverge from the rest of the market if geopolitical risks persist. Conversely, signs that shipping can move normally through the Strait of Hormuz could reduce the risk premium embedded in crude prices.

The verified facts support a cautious reading of Monday's close. Oil, yields, and energy stocks rose while nearly every other S&P 500 sector fell. August's strong index returns show that the market entered September with momentum, but the combination of inflation, Fed expectations, and Middle East conflict has materially raised the uncertainty surrounding that momentum.

Sources

• Associated Press: Oil prices rise and stocks fall after U.S. hits Iranian sites in the Strait of Hormuz
• Associated Press: How major U.S. stock indexes fared Monday
• CME Group: FedWatch

Market data reflects the U.S. regular trading session on August 31, 2026. This article is for informational purposes only and is not investment advice.