To seal the deal, the companies now have to cross potentially high regulatory hurdles in the U.S.--about a quarter of Syngenta’s sales come from North America—and elsewhere.

As well as antitrust authorities, a U.S. interagency committee known as CFIUS has the power to block deals that pose a threat to U.S. national security. Last month, Philips NV said it was terminating its $2.8 billion deal to sell its lighting-components and automotive-lighting business to Go Scale Capital, an investment fund led by Chinese venture-capital firm GSR Ventures.