The United States is a nation of diverse peoples bound together through an allegiance to the constitution. The Introduction looks at where the people of the United States have come from and how they have come together, whether through conquest, colonialism, the slave trade, the acquisition of territories, or voluntary migration. The United States did not become a nation by accident, but by choice. Since its founding in 1789, the United States has experienced almost constant immigration. There have been three notable waves: in the 1840s and 1850s, from the 1890s to World War I, and after 1965. There are many issues that Americans have to confront that relate to immigration.