The $19.6bn offer by China National Offshore Oil Corporation for Unocal, the US oil and gas group, is a landmark event. It is the biggest overseas bid yet by a Chinese company; it is the first to trigger a contested takeover battle and the first to be made in a politically sensitive “strategic” sector in the west.

For all those reasons, it is bound to arouse controversy in the US. Even before it was launched, Congress members were demanding it be vetted on national security grounds. The members nclude Duncan Hunter, hawkish chairman of the powerful House armed services committee, who opposed the ultimately successful takeover of IBM’s personal computer business by Lenovo - like CNOOC, a state-owned company.