US stocks were mixed on Monday, set for a retreat from a historic run of gains as President Trump's new tariff threat revived trade war worries in the wait for a Federal Reserve policy decision.

The S&P 500 ( ^GSPC ) slid roughly 0.3%, eyeing a cooldown from its longest winning streak in over 20 years . The Dow Jones Industrial Average ( ^DJI ) reversed early trading losses to rise 0.2%, while the tech-heavy Nasdaq Composite ( ^IXIC ) dropped about 0.5% to lead the declines.

Stocks are pulling back, and the US dollar ( DX=F ) is losing ground as Wall Street gauges whether recent confidence that Trump has moved to a dealmaking phase is misplaced.

Investors had been riding a wave of optimism, fueled largely by early indications that the US and China are inching toward the negotiating table for discussions on tariffs. Chinese officials have signaled reopening trade talks with Washington, though no start to negotiations appears imminent.

But Trump said at the weekend that he has no plans to speak to his Chinese counterpart, President Xi, this week, even as he said he wants a "fair deal" with the country.

In a social media post on Sunday, Trump said movies produced outside the US will face a 100% tariff . He said efforts to start the process will start immediately, though he offered little detail on the duties.

Looking ahead, Wall Street’s attention turns to the Federal Reserve's two-day policy meeting that kicks off on Tuesday. The Fed is expected to hold steady with current rates, even with Trump amping up pressure on its chair, Jerome Powell, in recent weeks.

Berkshire Hathaway ( BRK-B , BRK-A ) stock dropped about 3% after longtime CEO Warren Buffett said he will step down by the end of the year.

On the earnings front, this week sees earnings continue to roll in, with high-profile reports expected from Ford ( F ), Palantir ( PLTR ), Disney ( DIS ), and AMD ( AMD ) set to roll in this week.