Iran is no stranger to intense economic pressure. It has weathered U.S. sanctions for nearly four decades, and survived the brutal UN sanctions imposed over its nuclear program from 2010 to 2015. It learned how to avoid sanctions to sell its oil and acquire the products and raw materials it needed to sustain its economy. But the United States has also gotten more adept at cracking down.“Fifteen years ago some guy would set up a company in Dubai’s Jebel Ali port and load stuff from wherever and ship it to Iran and no one would notice,” Farhad Alavi, a sanctions-compliance attorney at the Akrivis Law Group in Washington, told me. “Now the detection systems and knowledge are leaps and bounds ahead.”

Despite Washington’s primacy in the global financial system and fine-tuned ability to detect suspicious transactions, Iranian traders will manage to make small deals, like purchasing food products from neighboring Turkey. But big firms will have a tough time purchasing specialized industrial products from Europe—say, machinery to manufacture automobiles, or the ingredients to produce pharmaceuticals. Banks around the world, many of which are connected via the European-based SWIFT system that allows them to move money, will be reluctant to finance or provide letters of credit for any dealings with Iran if it means they will be cut off from the United States. The restrictions also make it difficult to insure a deal or a shipment.

To circumvent the banking measures in the past, Iran has tried to conduct transactions in gold or in local currencies, allowing foreign companies to pay for oil and gas with their own money instead of dollars. China and India, for example, set up barter deals to do business with Iran using local currencies, while Turkey conducts some business with its neighbor in lira. The problem for Iran is that businesses abroad rarely want to be paid in Iranian rials, which are plummeting in value. In addition, the United States could demand that local currency or gold transactions also be limited, potentially foiling an ongoing attempt by Iran to build a trading network with Russia and Turkey. “Because Iran had problems with the U.S. central bank, Iran cut separate deals with Russia and Turkey,” Mehmet Koç, a scholar at the Center for Iranian Studies in Ankara, told me. “But the new condition Trump has put is that Iran can’t do business in either dollars or rials.”

In the past, Iran used a shifting network of front companies in places like China, the United Arab Emirates, Iraq, and Liechtenstein, to do business. Typically, these companies would operate by hiring outsiders as silent partners or registering under obscure names that didn’t immediately raise flags. For years, U.S. officials pored over financial records and intelligence intercepts to track down such firms. Starting during the Obama years, Washington needed only to issue stern warnings to governments and companies who wanted to emulate U.S. standards on corporate ethics and responsibility. “Companies are increasingly adopting a U.S.-style attitude toward compliance,” Alavi said. “Even if it might be legal they don’t want to go through all the risks.”