The city of Brussels is the present-day capital of Belgium and the de-facto capital of the European Union. Brussels has been part of the Roman and Frankish empires as well as under rule by the Hapsburgs, the Spanish, and the Netherlands. The Belgian Revolution occurred in 1830, which established Belgian independence with Brussels as its capital and Leopold I as the first Belgian king. German troops occupied Brussels during World War I and World War II. The 1948 Treaty of Brussels led to an intergovernmental alliance between Belgium, France, Britain, the Netherlands, and Luxembourg, which later became the North Atlantic Treaty Organization (NATO).