Lithuania. Largest country in Europe in the 14th century. Lost large sections of its territory to Sweden and Russia in the late 1600s and 1700s.
All of the Native American nations that were swept aside by the United States. Much of that was because of disease, but a lot was also because the U.S. was better organized and could pick off individual tribes one-by-one.
Mongolia. Genghis Khan's empire, once one of the largest on earth. Great fighters, poor administrators. Eventually overshadowed by China.