Despite Portugal's territorial claims in Angola, its control over much of the country's vast interior was minimal. In the 16th century Portugal gained control of the coast through a series of treaties and wars. Life for European colonists was difficult and progress was slow. John Iliffe notes that "Portuguese records of Angola from the 16th century show that a great famine occurred on average every seventy years; accompanied by epidemic disease, it might kill one-third or one-half of the population, destroying the demographic growth of a generation and forcing colonists back into the river valleys".