The S&P 500 rose to a new high on Friday, completing a swift recovery to a level last seen in February before expectations of a new business-friendly government gave way to widespread fears over the impact of trade tariffs.

It is a remarkable turnaround for a stock market that just a few months ago was being battered by investors’ fears that President Trump’s tariff proposals would bring chaos to global trade.

The S&P 500 has regained all the ground it lost in March and early April, when Mr. Trump proposed his broad array of tariffs and ignited concerns that the United States’ role as the world’s most dynamic and reliable economy was coming to a self-inflicted end.

The rally began on April 9, when Mr. Trump delayed his administration’s steepest tariffs until July. Since then, the S&P 500 has risen more than 23 percent, despite continued challenges like inflation and a war in the Middle East.

Mr. Trump’s decision to delay new tariffs cemented a perception among investors that the government would not let the stock market fall too far. The S&P 500 came within a whisker of entering a bear market — a marker of deep investor pessimism — the day before he said he would pause the tariffs while he sought deals with trading partners.

The president’s about-face — and the ensuing rally — has also validated investors’ strategy of “buying the dip” during the market’s sell-offs, hoping to reap big gains when it rises.