In the event a national monopoly forms such as a major railway or oil company, the U.S government (or any other government) can choose to take action such as breaking up that company into smaller companies. This would not be an option and that monopoly would have to comply.
However, let's imagine a scenario where there is a large international monopoly. Would the U.S government (or any other government) be allowed to force that international monopoly to break apart?