But as the focus on sanctions intensifies, a review of the United States’ experiences in enforcing its own longstanding restrictions on trade with Iran suggests it would be difficult to truly quarantine the Iranian economy.

Black market networks have sprouted up all over the globe to circumvent the sanctions…

“The Iranians have a lot of experience at this point in evading sanctions,” said Michael Jacobson, an intelligence and sanctions specialist at the Washington Institute for Near East Policy. “They are adaptable, learn from mistakes, see where the United States cracks down and move elsewhere. And on the part of businesses, there is a lot of willful blindness.”…

That kind of displacement, said Mr. Jacobson, the sanctions expert, shows that even strict sanctions, enforced with broad international support, would be unlikely to entirely seal off a country as large and complex as Iran. But, he said, the restrictions have a comparatively modest but nevertheless real benefit: they increase the cost and difficulty for Iranian elites of doing business, providing an incentive for the country’s leaders to negotiate.