The March 1 Trump tariff rout follows an ugly February for all three benchmarks.
"You get the algorithms responding to the technical break and initiating sell programs", said Ken Polcari, director of the NYSE floor division at O'Neil Securities in NY.
After oscillating between gains and losses through the session, benchmark U.S. indices ended in the red on Wednesday, snapping a streak of monthly gains for the first time in several months.
Campbell Soup, a big aluminium user, fell 0.4 percent, Coca-Cola lost 0.3 percent, while Budweiser maker Anheuser Busch InBev was about flat.
Indexes started higher on Wednesday but then waffled for much of the day before turning sharply lower in the last hour of trading.
Big exporters like Apple and drugmaker Pfizer, which would suffer if trade tensions picked up, also fell.
The S&P 500 increased 3.35 points, or 0.12 per cent, to 2,717.18.
United States markets have had worse days - this week, in fact - so a 71 point fall for the Dow could be counted as an acceptable outcome.
The tech-heavy Nasdaq composite index finished down less than 1 percent on Wednesday but is up more than 5 percent on the year.
Roger Bannister, runner who broke the 4-minute mile, dies aged 88
But Bannister and Australian rival John Landy as well as others were threatening to break it. It was as much of a psychological barrier as it was a physical barrier.
This Parkland Shooting Survivor Will Not Return To School At All
Claudia Ortiz said her daughter Daniella, 17, didn't want to come to orientation Sunday but is in school today. Despite all the bullshit that has been spewed about them, the students of Parkland have stood tall.
Maoists, Greyhounds Jawan killed in encounter on TS-Chhattisgarh border
The CPI (Maoist) received the setback when it was trying to revive its activities in its former stronghold of north Telangana . GreyHounds is a special force of the states of Telangana and Andhra Pradesh which undertakes operations against Naxals.
USA stock index futures gave up losses and were largely flat on Thursday after data showed consumer spending in January notched its smallest increase in five months and core inflation rose less than expected. For the month, the Dow fell 4.3% while the S&P shed 3.9% and the Nasdaq lost 1.9%.
The Dow Jones industrial average was little changed at 25,029.
In Canada, the S&P TSX Composite dipped 9 points to 15,385.
Bond prices rose. The yield on the 10-year Treasury note fell to 2.85 percent.
Wednesday was another down day for the broad USA markets with all three of the major exchanges closing lower on the day.
"The first quarter of 2018 will likely see the continuation of sales and margins in line with trends in the second half of 2017", added chief financial officer Lauren Peters. "He may be able to persuade his core voter base of that but investors are far from convinced, as was evident by the market reaction to the announcement [of import tariffs on steel and aluminum]", said Craig Erlam at Oanda.
Best Buy jumped around 5 percent after it reported strong same-store sales for the fourth quarter, recovering from falls overnight due to the US consumer electronics retailer's announcement of 250 small mobile phone store closures.
Dow futures are down 194 points, the index having closed 420 points lower yesterday, while S&P 500 futures are trading over 13 points lower.
The Dow Jones industrial average sank 77 points, or 0.3 percent, to 24,954.