GIOIA TAURO, Italy — From its headquarters in Calabria, the poorest, least-developed region in Italy, Gruppo Ventura looked out on the world and spotted a growth opportunity — Iran.

The family-owned company installs railroad tracks. The Italian economy was sluggish, while Iran was poised to develop rapidly, having promised to abandon its pursuit of nuclear weapons in exchange for relief from crippling international sanctions.

The deal that Gruppo Ventura struck last year with an Iranian partner was modest, but the possibilities seemed big. Then, President Donald J. Trump withdrew the United States from the Iran nuclear deal, dealing a blow to companies across Europe.

American sanctions not only bar domestic companies from doing business in Iran but also threaten foreign businesses with being frozen out of the American financial system.

Mr. Trump has also imposed tariffs on steel and aluminum, provoking outrage among European allies, while threatening to make steel more expensive.

Gruppo Ventura’s international expansion is now on hold.

«We were expecting to expand in Iran, build rails,» said Gruppo Ventura’s chief financial officer, Alessandro Ventura. «We are not expecting to do these things anymore.»

Only a few months ago, Europe was the leading example of the vigor the global economy was gaining after the trauma of a world downturn. Even Italy, Europe’s traditional problem child, was growing.

But Mr. Trump’s decision to walk away from the Iran nuclear deal threatens to cost European companies billions of dollars in lost sales, with German, Italian and French players especially exposed. The reimposition of sanctions on Iran stands to limit the flow of its oil to world markets. This prospect has lifted fuel prices, applying pressure to European economies.

And Mr. Trump refused to spare Europe from his tariffs on steel and aluminum, then used an annual meeting of major democracies to double-down on his clash with allies, enhancing fears of trade conflict.

Gruppo Ventura presents itself as relatively insulated against such shocks, given that the Italian government pays it to service rail tracks. Still, company executives are concerned that the cost of rail tracks will climb along with the price of steel.

«Everything is a bit more complicated,» said Gruppo Ventura’s chairwoman, Maria Antonietta Ventura.

All of this has been playing out against the latest outbreak of Italian political drama, which has placed power in the hands of two populist parties, the Five Star Movement and the League. They have promised to deliver tax cuts and a basic income program — unconditional cash grants for all — without elaborating on how they plan to pay for them, creating fresh worries about Italy’s alarming public debt.

On the afternoon when word broke that Mr. Trump would not exempt Europe from his metals tariffs, Claudio Capponi was struggling to calculate the impact. He is the commercial director of IRON, a publicly traded company that makes industrial parts.

«Europe is too good of a trade partner for the United States for President Trump to do this,» he said.

Given that IRON is a buyer of steel, the company might benefit from the American tariffs. Steel now shipped to the United States from mills within Europe might stay here to avoid the tariffs, raising the supply and lowering prices locally even as they increase in much of the world.

But Mr. Capponi believes his customers are likely to squeeze him for lower prices.

Also, Mr. Capponi said, «We depend on access to a global market. It’s the uncertainty that is driving us mad. We are looking out the window at what is going on outside of our factory. We’re worried about the precedent of these tariffs. We are worried the forces of protectionism are being set loose.»

It was all humming along nicely. Then along came Trump.

© 2018 The New York Times