US Treasury Secretary Scott Bessent's threat of an “economic D-Day” against Iran could put Washington on a collision course with China, raising questions over how far the Trump administration is prepared to go against Tehran's most important economic partner. China is by far the biggest buyer of Iranian oil and provides a crucial channel for Iran to remain connected to international markets despite US sanctions. The threat comes as President Donald Trump seeks to wind down his unpopular war in the Middle East while maintaining economic pressure on Iran.

Bessent on Monday announced a fresh package of sanctions targeting dozens of entities, individuals and vessels linked to Iran. However, the more consequential part of his announcement was the warning that Washington could pursue secondary sanctions against companies and countries that continue doing business with Tehran. Secondary sanctions would potentially widen the impact of US measures beyond American companies and institutions. Businesses and financial entities in other countries could face penalties for continuing commercial relationships with sanctioned Iranian entities.

Such a strategy could significantly restrict Iran's ability to sell oil, access financial channels and maintain trade with international partners. China presents the biggest challenge to that approach. Beijing purchases the overwhelming majority of Iran's oil exports, with estimates cited in the report putting its share at around 90 per cent. Iranian oil is therefore closely tied to China's energy demand, while China's purchases provide Tehran with one of its most important sources of revenue under the existing sanctions regime.

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Any serious attempt by Washington to target companies involved in China's purchases of Iranian oil could consequently create tensions between the world's two largest economies. The United States and China have been attempting to maintain a fragile trade truce, and Washington has an interest in preventing new measures from triggering another round of economic retaliation or destabilising global markets. Bessent's warning is also not entirely new. The Trump administration has previously threatened tougher enforcement against businesses and countries continuing to trade with Iran.

The key question is whether Washington is now prepared to enforce those threats against major Chinese entities, particularly when doing so could have consequences extending well beyond the US-Iran relationship. The Treasury secretary's latest announcement indicates that the administration intends to increase pressure on Iran's financial networks. By targeting individuals, companies and vessels, US authorities can attempt to make it more difficult for Tehran to move money, transport oil and conduct international transactions. However, the effectiveness of such measures depends heavily on the willingness of major trading partners to comply.

For Iran, China's continued purchases of its oil are particularly important because they provide a substantial outlet for exports despite years of US sanctions. If Chinese buyers were significantly restricted or deterred, Tehran could face additional pressure on its revenues and broader economy. Conversely, if Beijing continues purchasing Iranian crude despite Washington's warnings, the US would face a difficult choice over whether to impose penalties that could directly affect Chinese businesses. The issue also has implications for global energy markets. Any major disruption to Iranian oil exports could affect supply expectations and potentially contribute to higher crude prices, particularly if sanctions are accompanied by further geopolitical tensions in the Middle East.

Washington therefore has to balance its objective of squeezing Iran with the broader economic consequences of aggressive enforcement. Bessent's “economic D-Day” warning has thus raised a question larger than the future of US-Iran sanctions. Its real test could be whether Washington is willing to confront companies and financial interests in China that continue to support Iran's oil trade. The administration's ability to apply pressure without damaging its relationship with Beijing or causing a wider economic shock will determine how far the latest sanctions campaign can go.

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