A day after stunning markets, Republican lawmakers and even his own advisers by announcing stiff tariffs on steel and aluminum imports, President Trump doubled down on his approach on Friday, saying in an early morning tweet that “Trade wars are good, and easy to win.”

Mr. Trump appeared eager to defend his decision to levy sweeping tariffs on all imports of those metals, issuing a series of morning tweets explaining the need for tariffs. “Our steel industry is in bad shape. IF YOU DON’T HAVE STEEL, YOU DON’T HAVE A COUNTRY!” he said in one tweet.

Markets fell in response to Mr. Trump’s announcement on Thursday that he would impose tariffs of 25 percent on steel and 10 percent on aluminum, effectively placing a tax on every foreign shipment of those metals into the United States. Mr. Trump, at a hastily arranged meeting with industry executives on Thursday, said he would formally sign the trade measures next week and promised they would be in effect “for a long period of time.”

On Friday, he wrote that the measures would help to reduce the trade deficit, which is the gap between what the United States exports to other countries and what it imports. Mr. Trump has long lambasted the trade deficit as a sign of United States weakness.