US sanctions are designed to act like a virus: they spread, so that eventually nobody in the world is allowed to do business with the target country, unless the

US sanctions are designed to act like a virus: they spread, so that eventually nobody in the world is allowed to do business with the target country, unless they want to completely cut themselves off from all economic interactions outside the target country. A European company can't do business with Iran, because it does business with a European bank, which does business with another European bank, which does business in the US. A Chinese chip foundry can't do business with Huawei, because the foundry buys equipment from a Japanese company, which uses a piece of software written by an American company somewhere in its pipeline. The global economy is tightly interconnected, and the US takes advantage of this fact to cut companies or even entire countries off from the global economy. As far as I'm aware, no other country uses secondary sanctions anywhere near as aggressively as the US does.