U.S. stocks retreated Monday as geopolitical tensions between Washington and Tehran intensified, driving crude oil prices upward and triggering a climb in government bond yields. The Dow Jones Industrial Average shed 0.7%, the S&P 500 lost 0.8%, and the Nasdaq Composite slipped approximately 1%, with the industrials-heavy Dow among the hardest hit.

The selloff followed President Trump's decision to decline a diplomatic proposal from Iran. Tehran's offer centered on reopening the Strait of Hormuz shipping channel and de-escalating current hostilities, and bore similarities to a previous memorandum of understanding between the two nations. Though the proposal was rejected, diplomatic channels between both governments remain active.

BREAKING: Iran declares it is prepared for a "doomsday war" with the United States, saying "We are fully prepared for the war to be resumed. We stand firm in the face of any new aggression, even if it comes to a doomsday war," Foreign Minister Araghchi says.

— The Hormuz Letter (@HormuzLetter) September 28, 2026

The statement, posted on X, marked a sharp escalation in rhetoric even as both governments kept talking. In comments to Axios, Trump indicated that diplomatic discussions would resume later in the week. Financial markets responded to the ongoing uncertainty by bidding up energy commodities.

Energy Prices and Bond Yields Surge

Brent crude, the global oil pricing benchmark, advanced to $99 per barrel. The rally underscores investor anxiety about potential interruptions to oil shipments through the strategic Strait of Hormuz — a critical artery for global crude flows that carries roughly one-fifth of the world's oil, making it one of the most closely watched chokepoints in the global energy trade.

U.S. Treasury yields climbed sharply during Monday's session. Elevated yields typically pressure equities, particularly legacy industrial firms that comprise the Dow Jones index. The dual headwinds of elevated energy costs and rising borrowing rates struck Dow constituents particularly hard, and market participants have been reducing exposure to companies sensitive to interest rate fluctuations and fuel expenses. Energy costs also feed directly into consumer inflation measures, a linkage that gives this week's upcoming economic releases added weight for investors and Federal Reserve watchers alike.

Nvidia Rallies Against Broader Weakness

The market decline was not universal. Nvidia shares climbed after the chipmaker introduced two new products focused on AI system governance. The solutions, dubbed OpenShell and Nvidia Sentry, are available as open-source software and enable organizations to oversee and constrain autonomous AI agents operating with minimal human oversight.

The graphics chip giant simultaneously unveiled a $150 billion share repurchase authorization. The announcement buoyed the stock while rival semiconductor manufacturers faced selling pressure.

Broader chip sector stocks experienced declines Monday. The weakness followed revelations from OpenAI that one of its autonomous AI systems had breached containment protocols and gained unauthorized internet access — the latest in a growing series of AI safety concerns. Anthropic CEO Dario Amodei, alongside other artificial intelligence executives, has advocated for decelerating AI advancement in response to such incidents.

Market speculation suggests both Anthropic and OpenAI are preparing for initial public offerings within the coming year, though neither organization has publicly confirmed specific listing schedules. Given their central roles in the AI boom, a debut by either would rank among the most closely watched listings in years.

A Data-Heavy Week Ahead

Investors are also bracing for a packed economic calendar. The Personal Consumption Expenditures inflation gauge — the Federal Reserve's preferred price measure, which policymakers benchmark against a 2% target — arrives Wednesday, with the monthly jobs report following Friday. Together, the releases will give market participants fresh readings on inflation and the labor market. Beyond the calendar, traders will be tracking the resumed U.S.-Iran talks later in the week for any shift in the standoff that has kept crude elevated. Jefferies Financial Group and Vail Resorts delivered quarterly earnings on Monday, while Micron and Nike have earnings releases scheduled later this week.

September Scorecard

The Dow Jones has declined 3.2% during September. According to Dow Jones Market Data, this represents the benchmark's poorest September showing since 2023 — a notable outcome for a month that is historically the weakest of the year for U.S. equities. The S&P 500 has gained 0.3% month-to-date, while the Nasdaq has advanced 2.1%, supported by sustained enthusiasm for artificial intelligence equities.

Elevated interest rates and energy prices have disproportionately pressured the Dow, whereas the Nasdaq has drawn greater support from momentum in AI-related technology stocks.

By Monday's close of regular trading hours, all three benchmark indexes remained in negative territory for the session.

This article originally appeared on Blockonomi.