With more than 80% of global trade by volume being transported by sea, maritime shipping lanes are indispensable to the world economy. That fact was starkly illustrated by the war in Iran, which saw Tehran effectively close the Strait of Hormuz to commercial traffic and Washington respond with a blockade of Iranian ports.

Yet such recent events are an aberration from much of the post-Cold War period, during which economic sanctions were enforced far from the sea. Governments relied on financial infrastructure – bank messaging systems, insurance markets, shipping registries and port access rules – to restrict trade without physically stopping ships.

But that system is now under strain. As the United States and its partners have relied more heavily on sanctions as a tool of geopolitical conflict, targeted countries have developed effective evasion networks. In response, the U.S. and its partners are increasingly returning to a more direct form of economic pressure: boarding ships at sea.

Since late 2024, naval forces in Europe and among NATO partners have detained or inspected numerous vessels suspected of carrying sanctioned cargo. These operations have focused on so-called shadow fleet tankers transporting Russian oil. Since the U.S. blockade of Venezuelan oil began in late 2025, interdiction has spread beyond Russian-linked tankers to Iranian and Venezuelan vessels, and now European and Indian authorities have joined in that effort. These ships often operate in legal gray zones, using opaque ownership structures, frequent flag changes and alternative insurance arrangements to avoid sanctions enforcement.

As a longtime observer of international security and geopolitical risk, I believe this trend suggests not a coordinated global policy but a broader shift in practice: Sanctions enforcement is moving from financial systems back into physical space.

Why financial sanctions are losing leverage

Modern sanctions have long relied on control over key nodes in global commerce. U.S. and European sanctions on Iran and Russia show how restrictions on dollar clearing, the SWIFT banking network and maritime insurance can severely disrupt trade........

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