If you take over your neighbor's backyard and claim his in-ground swimming pool as your own, you might seize control of, or usurp his yard, but he'll probably call the cops on you.
Leaders who usurp power don't ask for permission to take control of their country. They seize power, often with the help of a large army of followers. A usurper doesn't have to be human. A brand-new radio station can usurp the most popular station in town by playing a better mix of music.