WASHINGTON—President Trump walked back his steep tariffs on nearly 100 nations that had taken effect just hours earlier, a shocking reversal that sent stocks soaring even as Trump announced simultaneously that he would raise levies on Chinese imports to 125%.

The bond market movements raised fears that investors were ditching U.S. government bonds amid a gathering panic over Trump’s assault on global trade.

For weeks, President Donald Trump has plowed ahead with plans to impose steep new tariffs on imports despite warnings that the policy could trigger economic chaos. On April 9, after a multiday plunge in global stock markets,