US stocks mostly declined on Monday as investors navigated a fresh wave of trade tensions, impending sanctions against Iran, and a pivotal week ahead for technology earnings.

The S&P 500 dropped 0.3% and the Nasdaq Composite fell 0.8%. The Dow Jones Industrial Average was the only major index to advance, gaining 0.3%.

US-Canada Trade War Heats Up

Trade negotiations between the United States and Canada broke down over the weekend after the US imposed new tariffs on Canadian goods. Canadian Prime Minister Mark Carney suspended the talks and pledged to respond with matching tariffs.

President Trump then threatened to raise tariffs on Canadian autos, auto parts, and steel to 50%, starting January 1, 2027.

BREAKING: President Trump says tariffs on all cars, trucks, automotive parts, and steel from Canada will be increased to 50% effective January 1st.

"Canada will be treated like a state no longer," Trump says. pic.twitter.com/c8zbtMJzm2

— The Kobeissi Letter (@KobeissiLetter) August 24, 2026

Shares of General Motors and Ford both moved lower on the news. Canada is one of the largest sources of vehicles and auto parts imported into the United States, and North American auto supply chains are deeply integrated, with components often crossing the border several times before final assembly. The auto tariff threat added to an already uneasy mood in markets, with investors heading into a week packed with major events.

Chip Stocks Lead the Decline

Technology and semiconductor stocks took the brunt of Monday's selling. Sandisk and Micron fell after reports that Apple is testing chips made by China's CXMT. CXMT, formally ChangXin Memory Technologies, is China's leading maker of DRAM memory chips, competing in a market long dominated by Micron, Samsung, and SK Hynix — the suppliers Apple has historically leaned on for memory.

Alibaba also announced a $10.2 billion share sale to fund its artificial intelligence push, adding further pressure to the semiconductor sector. The company has previously committed more than 380 billion yuan (about $50 billion) over three years to AI and cloud infrastructure, one of the largest such budgets among Chinese technology firms.

Iran Sanctions Add to Market Pressure

The US is preparing a new round of economic sanctions against Iran. Treasury Secretary Scott Bessent is expected to provide more detail Monday afternoon.

In a Financial Times op-ed, Bessent described the measures as the largest financial offensive ever launched against an adversary. Markets were watching closely for specifics. Iran is a significant oil producer within OPEC, so any measures that curb its crude exports are a key variable for energy markets.

In bond markets, the 10-year Treasury yield dipped to 4.72%, while the 30-year yield fell to 5.24%.

Nvidia Earnings in Focus

All eyes are now on Nvidia, which reports earnings Wednesday. The company has become a symbol of the AI trade, and its results are expected to set the tone for the broader market. Nvidia is the world's most valuable chipmaker and among the largest companies by market value; its data-center GPUs power most large-scale AI systems, making its results a widely watched proxy for AI infrastructure spending by cloud giants such as Microsoft, Amazon, Alphabet, and Meta.

The iShares Semiconductor ETF was down again Monday. The index had surged into a new bull market earlier this month but has faced renewed selling pressure over the past week.

Chris Larkin, managing director at E*TRADE from Morgan Stanley, said Nvidia and other tech earnings are "positioned to be a major weight on the market's momentum scale."

Federal Reserve Chairman Kevin Warsh is also scheduled to speak Friday at the Jackson Hole summit, the Fed's annual retreat in Wyoming, where chairs have historically used keynote addresses to telegraph shifts in policy. Bond traders will be watching his comments closely for signals on interest rate policy.

The combination of tariff news, Iran sanctions, and the Nvidia earnings report makes this one of the busier weeks of the year for market watchers.