China's state-owned Cnooc Ltd. (HKG:0883) has cleared a key regulatory hurdle -- something it failed to do seven years ago -- on its way to making what will be the nation's largest-ever foreign acquisition.

The Committee on Foreign Investment in the United States (CFIUS) approved the $15.1 billion acquisition of Nexen Inc. (TSE:NXY), a Canadian oil and gas concern that holds drilling and production rights in the U.S. Gulf of Mexico as well as interests in oil sands in Alberta. The deal is expected to be closed by the end of the month, Reuters said Wednesday.

The approval stands in sharp contrast to the refusal in 2005 by CFIUS to approve Cnooc's proposed $18.5 billion purchase of Union Oil Company of California, known as Unocal. That company was bought instead by Chevron Corporation (NYSE:CVX) for $17.9 billion, $600 million less than Cnooc was offering.

The question of why the U.S. had to approve the deal can be found in Nexen’s offshore assets in the oil-rich Gulf of Mexico. There’s a general mistrust of state-involved interests buying assets on U.S. territory, and the U.S. deems it a matter of national security when deals like this emerge.

Despite lingering “strategic mistrust” between the U.S. and  China, this week's approval could be a sign that, at least in mega-deals like this, the position is softening. In January, the CFIUS signed off on China auto parts manufacturer Wanxiang Group Corporation’s acquisition of bankrupt lithium- ion battery maker, Waltham, Mass.-based A123 Systems, Inc. (PINK:AONEQ).

A123 also had military contracts. To allay concerns about that, and to lessen the chances the U.S. would block the acquisition, Wanxiang sold A123’s military projects to Navitas Systems, LLC.

“The bigger issue here was the sensitive nature of the programs,” Thomas Golab, vice president for strategic initiatives at Illinois-based Navitas, told The Financial Times.