US equity indices declined sharply on Monday, erasing gains from the previous session as renewed uncertainty over US trade policy prompted investors to cut exposure to risk assets. The S&P 500 and Nasdaq 100 each fell 0.7%, while the Dow Jones Industrial Average dropped more than 600 points. President Trump invoked Section 122 emergency economic powers to impose a 15% tariff on all countries, prompting major trading partners to suspend the implementation of previously agreed trade deals with the US and heightening uncertainty over how the administration might seek to prolong the tariffs given likely resistance in Congress. The moves came on the heels of a Supreme Court ruling that struck down tariffs imposed on certain countries since April of last year. Large software and payments companies also came under pressure amid renewed concerns about AI-driven disruption, with Oracle and Palantir each falling 4%, and American Express sliding 7% to weigh heavily on the Dow. Nvidia edged higher ahead of its earnings report later this week.