Recall the scene back in 1981 when a deranged John Hinckley shot Ronald Reagan in a botched assassination attempt in front of a Washington hotel. Reagan was rushed to the hospital and made a speedy recovery. In the immediate aftermath of the shooting, then Secretary of State Alexander Haig infamously went on television and asserted that he was “in control here” at the White House (pending the return of Vice President George H. W. Bush, who was traveling outside of Washington.) In reality, Alexander Haig was not legally in control of anything; at best, he had de facto control of the White House, meaning that he had practical control but not lawful or legitimate control.
The phrase de facto is normally used to characterize an officer, an action, or some state of affairs which must be accepted for practical purposes but is, in fact, illegal or illegitimate. Contrast this with an officer, an action or a state of affairs which is de jure, which means lawful, legitimate and proper.
If someone is described as the de facto ruler it should not necessarily be taken as a compliment. It simply means that his or her leadership is a matter of fact even though possibly subject to challenge or not fully accepted by the people.
— BBC&B