In the late 19th century; Imperial Germany conquered the regions that are now Tanzania (minus Zanzibar); Rwanda; and Burundi; and incorporated them into German East Africa. During World War I; an invasion attempt by the British was thwarted by German General Paul von Lettow-Vorbeck; who then mounted a drawn out guerrilla campaign against the British. The post-World War I accords and the League of Nations charter designated the area a British Mandate; except for a small area in the northwest; which was ceded to Belgium and later became Rwanda and Burundi.