If a business transaction could result in foreign control of a US business, the US government has the power to review and take action if national security so requires. There is a process for voluntary pre-transaction submissions to the US government which is highly recommended, to avoid possible post-merger problems. This process involves a unit of the US Treasury Department which is called the Committee on Foreign Investment in the United States. The usual result of their review may be an announcement that the government is not choosing to exercise its power to review or otherwise act concerning the transaction. 

Note that there is no small business exception to this review, and that joint ventures which create control over a line of business also trigger security review. Passive investments, loans, or investments of 10% or less in a business do not trigger control transactions review. 

Instructions for advance submissions, contact information, copies of regulations, and details on changes in the process in recent years are available at http://www.treasury.gov/resource-center/international/Pages/Committee-on-Foreign-Investment-in-US.aspx