The falls mark a break from a period of low volatility and subdued moves. That is to say nothing of the fights against the Islamic State militant group in Iraq and Syria, and the continued movement of USA troops and military equipment along Eastern Europe in a posturing war with Russian Federation.
"You've elected a Republican administration which should be good news for the market, good news for business, and they're getting precious little done". That is why stocks are selling off. "Anything can be an excuse", he said.
Markets also remained on edge after a terror attack in the center of Barcelona killed at least 13 people and wounded more than 50.
US Treasury yields fell as investors, unnerved by the deadly attack in Barcelona and Washington turmoil, favored safe, low-yielding bonds over stocks and other risky assets.
Also, when rumors emerged Thursday about a possible resignation of the chief economic adviser of the White House, Gary Cohn, the markets are a little more alarmed.
Concerns have grown too over Trump's ability to push through his economic goals such as tax cuts and infrastructure spending, following the exodus of executives from two prominent business councils in reaction to his response to clashes last weekend in Charlottesville, Virginia.
The exodus of executives sparked talk that Mr Gary Cohn, Mr Trump's top White House economic adviser and a key liaison to the U.S. business community, might resign in protest.
The S&P 500 was on pace for its worst back-to-back weeks since the election.
Shanghai, Sydney and Seoul all fell, just as Asia-Pacific stocks had clawed back much of their losses from last week's slump over fears of military clashes between North Korea and the USA, fuelled by angry threats from both sides.
Small-cap stocks also sold off. The Russell 2000, meanwhile, closed below its 200-day moving average.
Advancing issues outnumbered declining ones on the NYSE by a 1.25-to-1 ratio; on Nasdaq, a 1.16-to-1 ratio favored advancers.
Trump, a real estate magnate who had never before held public office, was elected President in November touting his experience in the business world and ability to strike deals.
Rather than a single catalyst, a range of worries prompted investors to take profit, including the USA relationship with North Korea, the Barcelona attack, as well as domestic turmoil, according to Dennis Dick, proprietary trader at Bright Trading LLC in Las Vegas.