In the early days of colonization, many European settlers were subject to food shortages, disease, and attacks from Native Americans. Native Americans were also often at war with neighboring tribes and European settlers. In many cases, however, the natives and settlers came to depend on one another. Settlers traded for food and animal pelts; natives for guns, tools and other European goods. Natives taught many settlers to cultivate corn, beans, and other foodstuffs. European missionaries and others felt it was important to "civilize" the Native Americans and urged them to adopt European agricultural practices and lifestyles. However, with the increased European colonization of North America, the Native Americans were displaced and often killed. The native population of America declined after European arrival for various reasons, primarily diseases such as smallpox and measles.